I've never had issue with the upgrade process directly. It's fast and painless.   The upgrade doesn't actually do anything with mail files, just code.  However, there are differences in settings and some default behaviors that required configuration changes along the way.  Spam filtering has been significantly changed, for instance. 

The change-logs are a good place to start.
ben

On 12/01/2010 12:55, Black Night Lists wrote:

Hi All,

I will be upgrading our Surgemail server for the first time this weekend. I of course will ensure all files are backed up just before the upgrade happens in case I need to rollback but has anyone had any issues post upgrade. (I will be going from 4.2 to 5.1c)

Regards

Stephen Thirlwell
Black Night Software
Lasso Hosting and Development
http://www.ukmachosting.com