How should I clean up old mailbox files?

I'm using Mail.app to read my gmail via IMAP. I recently realised that, since I have a huge amount of mail in that account, and a fairly small SSD, it would be wise to reconfigure the account the save as much storage space as possible. I've done that, but it seems the old mailbox file hasn't been removed. In particular,

~/Library/Mail/V2/IMAP-{email account}/[Gmail].mbox/All Mail.mbox

is massive: 2.9G. I believe that's a hangover from the old account that I've removed. I'm unsure whether everything else in the [Gmail].mbox is new or old, but it's all much smaller.

There's also a directory named:

~/Library/Mail/V2/IMAP-{email account}/INBOX.mbox which is much smaller (16M) and I believe represents the account I've now configured in Mail.app.

What's the best way of clearing this all up? Should I try to selectively rm individual directories, or should I just remove everything and start over? Is there a nice way to remove everything (or, ideally, purge the stuff not in use) or should I rm the directories in V2 that relate to specific email accounts (I have other accounts in use, but I'm happy to remove them and set them up again if recommended)? I'm wary about ending up with a non-functioning Mail.app - if there's a way to uninstall (including data) and reinstall the app, that might be the best option.