What is this blog post about? It looks interesting...
http://google-cz.blogspot.com/2008/01/gadget-pro-pevod-email-t-zdo-google.html

Can you migrate from one GApps to another?

Yeah, there are quite a few apps that allow that:

http://www.google.com/enterprise/gallery/apps/migration.html

The one in the screenshot is gXFER:

<< gXFER is a simple, one click utility that allows users to move email, contacts and calendar. gXFER enables the transfer between any combination of Gmail and Google Apps Gmail accounts: Gmail to a Gmail, Gmail to Google Apps Gmail and vice versa, and Google Apps Gmail to Google Apps Gmail. The user simply enters their credentials for each account, and gXFER does the rest. >>

I think you need Google Apps Premier Edition account for it to work though.

Can recommend gMove to upload historic emails to gmail – and yes you do need a premier account. Fortunately they have a 30 day trial period – which i cancelled after i had completed the 5Gb upload (which took a few days).

It is great to be able to check emails which go back 8 years without being at home. And to be extra safe I also used gXfer to move a copy of the emails to a standard gmail account as well.

<<Can recommend gMove to upload historic emails to gmail>>

You can also use IMAP.

> You can also use IMAP.

Possibly not a good idea for 5GB of emails though, given what Google says here:

<< My account has been locked

If we detect abnormal usage that may indicate that your account has been compromised, we may temporarily disable access. It will take between one minute and 24 hours for access to be reinstated, depending on the behaviour detected by our system.

Unusual account activity includes, but is not limited to:

   1. Receiving, deleting, or downloading large amounts of mail via POP or IMAP in a short period of time. >>

http://mail.google.com/support/bin/answer.py?answer=43692

The use of the API on premier apps allows for the upload – avoiding the problem.

Alternatively use an "alias" gmail/apps account to get the emails onto the google platform and then transfer using gXFER (this uses a POP account). For 5Gb it took about 3-4 days.

Tony:
Nobody forces you to upload all the messages at once. But the question is: what's the limit?