Background
Back in 2004, I was still in high school, and choose a nickname for my email address to GMail. I am now out of college and using that same email address for business correspondence. Alas, the nickname itself isn't professional.
For years, I used a second email address from my nickname address. Any mail sent to "wattslevi" was forwarded to my nickname, and my nickname account was set up to send email as the professional name. Part of the "email as" feature tells the receiver who actually sent the email.
"From: nickname@gmail.com [mailto:professionalname@gmail.com] On Behalf Of Levi Watts"
I dreaded the process of moving over to another account. I found the process to be much easier than I expected, even after considering the non-email aspects of my account (Google Docs).
The Process
GMail: Getting all of your old accounts emails into the new account
From the old account, go to "Settings" in the top right corner of GMail. Select the "Forwarding and POP/IMAP" tab, and enable under POP Download: "Enable POP for all mail". Click "Save Changes" below after that. Note: If you have previously used POP on this account, the wording may be slightly different.
From the new account, go to "Settings" in the top right corner of GMail. Select the "Accounts" tab, and under "Get mail from other accounts", click "Add a mail account you own." You will need to provide Google with your email address and the credentials for your account. Leave the POP Server and Port numbers alone. The "Leave a copy of retrieved message on the server" must be left off (GMail will provide an error saying it isn't allowed).
The other options in the popup are as you choose. I choose to leave the SSL security on as I'm sure I have personal information within my thousands of messages over five years. I also choose to archive the messages, with the intent to turn that function back off after all my emails got here.
Depending on how much mail is in your old account, it will take at least a half day for all those messages get transferred. After about five minutes, check to see if you have messages coming in (they will be oldest to newest). If you choose to archive your old messages, you will need to select "All Mail" to see the progress. Alternatively, you can also see its progress within "Settings" -> "Accounts" -> "Get mail from other accounts".
Your new account will continue grabbing 200 emails at a time until all the mail is retrieved. Some groups of 200 will take longer than others, depending on how big attachments to those messages are. The number of remaining emails will likely be displayed wrong. Its never told me I had more than 400 emails left, and I had about 4000 emails come across. In my case, it took the tool about 16 hours to retrieve 5 years worth of emails, and cost me about a half hour to learn to set it up.
About an hour into the process, a red error box appeared under the account settings letting me know that there was a problem getting email. From the "View history" of that account, I was told the error was on GMails side. It wasn't too long before it began to continue working again.
Drafts, sadly, must be taken across by hand. For my 52 drafts, this still hasn't been done.
Google Docs: Getting ownership of your old files
From Google Docs, select "All items", click on "Select: All ##" at the bottom of the screen, then select "More actions" -> "Change Owner". You will likely be greated with a "You do not have permission to change owner of x of the items selected." After clicking ok, any documents you weren't allowed to give away will be deselected, and you can hit "More actions" -> "Change Owner" again.
Provide your new email address, and select "Change owner." This is one of those bad times to spell your email wrong. After doing this, I was greated with an error saying something went wrong. Ignore it, and leave that page up. Check to see if your new account is getting new documents, and check again in five minutes. For me, that was enough time for all my documents to change owners (even though I had the error message).
At time of writing, the owner of spreadsheets can't be changed.
Google Calendar: Getting your old accounts calendars
I couldn't find a function to give a calendar away, but from the old account, you can make the new account a co-owner. In the top right of Google Calendar on the old account, go to "Settings", then hit the "Calendars" tab, then for each calendar, hit "Share this calendar", and under "Share with specific people" add your new account's email address and select "Make changes AND manage sharing.
I am fairly certain this gives you just about all the same rights over the calendar as you would if you owned it proper.
Another option is to Export your calendars from the old account and Import them to the new account. This option is found from "Settings" -> "Calendars" -> and just below the "Create new calendar" button.
As far as I know, Task isn't designed to be given from one person to another. You will have to manually move your task over. Hopefully there aren't too many.
Picasaweb: Moving your web album from one account to another account
Currently, there isn't a simple means of moving your pictures from one account to the other. The easiest method is to use Picasa and upload to a new account the same as you did for the old account. I'm fairly certain you will lose any captions you set on the web version of your album.
I don't believe the work is worth it for how often I use this part of my account.
Google Reader: Getting all those subscriptions from one account to another
From the Google Reader on the old account, go to "Settings", and select the "Import/Export" tab, and choose the "Export your subscriptions as an OPML file." You will be given a file named "google-reader-subscriptions.xml."
From your new account, go to the same tab, and select "Choose File" and click Upload. You will loos what items you have and haven't read, but you will have your feeds.
Google Maps: Giving your new account access to your maps
From Google Maps on the old account, select "My Maps", and for each map you want shared, select it, and select "Collaborate" on the left just below the map selection section. Add your new account as a collaborator.
For each received email making your new account a collaborator, you will need to select the link to "accept" the map.
This will be tedious to people who have many maps in their account.
Other services
The other services of Google, I do not use often enough to find it useful to change which account they function under. This blog, for instance, will remain under my nickname account.
If you go though the hassle of figuring out how to transfer another Google service from account to account, I encourage you to write your own blog entry, and link to your blog in the comments of this entry.
One last note
To see all the services you have used with Google, please see:
https://www.google.com/accounts/ManageAccount?hl=en_US