As stated here (and as i checked), to upload a new version of an installed webpart, uninstalling the assembly from the GAC and installing the new version would suffice. I've tried it and it worked.
Now, in my development environment I have have a new version of my webpart. In production, I uninstall the .dll from the GAC, recycle the application pool and if I reload the page, the webparts are not shown. This is expected.
I then install the new version (directly taken from development environment's GAC), recycle again the pool, and the webparts are back on SP pages but those are the old versions! I've checked the browser cache and I've also performed an iisreset on the server, but nothing seems to work.
Where is SP taking the older version?