I wanted to let everyone know my experience with migrating - I know these forums are the first place I look before doing anything so I wanted to pay it forward.
It went well, though there were some hairy moments.
I was finally able to move our hosting from php5.2 to a higher version and eager to upgrade Piwik, to 2.8.3 in my case.
I went for automatic upgrade and soon got “fatal error on line 106 of core” something. Ok don’t panic - clicked back and asked to log in “Error: You can’t access this resource as it requires a ‘superuser’ access” and didn’t recognise my username and pw.
Panicking a bit now.
I refreshed the window and I got back the installation screen with “Your Piwik database is out-of-date, and must be upgraded before you can continue.
Piwik database will be upgraded from version 1.12 to the new version 2.8.3.
The following plugins will be updated: DevicesDetection.
The following dimensions will be updated: log_visit.user_id.”
Sounds ok and off I go, until “Internal Server Error
The server encountered an internal error or misconfiguration and was unable to
complete your request.
Please contact the server administrator, and inform them of the time the error
occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an
ErrorDocument to handle the request.”
No idea what I’m doing now, so I just refresh and I’m back to the installation screen “
The following dimensions will be updated:
log_link_visit_action.idaction_content_interaction,
log_link_visit_action.idaction_content_name,
log_link_visit_action.idaction_content_piece,
log_link_visit_action.idaction_content_target.”
Click go again and it complete successfully, log in and I’m all set!! 
Everything seems ok, but I’ve noticed that my database size has gone from 660MB before to 418MB after - is that normal?
Also, 2 hours after the upgrade it now seems to increasing in size quite quickly, every minute or so - maybe it’s reprocessing something?
Any suggestions about what went on with the size decrease?