hi
what do you mean by "upgrading web parts"? If you have some web part (visual web part, which is just lightweight wrapper over user .ascx control, or if rendering logic is implemented in c#) and want to update its logic, then you need to recompile the assembly
with the code of the web part and update it in the GAC of each web front end server in your farm (alternatively you may re-build wsp package with this assembly and re-deploy it to the farm. It will update assembly in the GAC automatically on all WFEs) and
make iisreset on all servers.
Blog - http://sadomovalex.blogspot.com
CAML via C# - http://camlex.codeplex.com
