http://www.hogbaysoftware.com/products/simpletext
What you should do:
1. Login with simpletext to your old Google account.
3. Copy the text files from your Mac's local SimpleText folder that
are synced down to your computer desktop.
4. Now logout SimpleText for Mac, and login with your new Google account.
6. Copy the files from your desktop into your newly synced SimpleText
folder on your mac.
I think that should do it, but keep backups of your files until you
are sure that they got copied to the new account.
Jesse