Hello,
Oh yes the dreaded move from 2.1.2 to 3.x!
The first thing you need to know is that if you want to upgrade DNN, you need to upgrade to DNN3.x rather than 4.x as 4.x will be too big a jump at this stage. (DNN4.x runs in ASP.NET2.0 and 3.x runs in ASP.NET1.1) - They are both the same currently apart from that. (although 3.x is currently more stable)
DNN3.x brought in some breaking changes, so the problem you have is that you will need to create a separate DNN3.x installation on a localhost installation and test each of your 2.1.2 modules in a DNN3.x installation.
Unfortunately I can't remember what the breaking changes were, I had notes on it at the time, but after some looking around in my files I can't find them. - I would look around the asp.net DNN forums for that info as there was quite a bit of discussion on it at the time.
To be honest, I have several DNN2.x sites that are stuck in 2.x because I spent a year developing modules for the sites and I didn't have the time spare to do the required changes to DNN3.x so I don't have any experience converting across.
All of the main Modules Developers will have experience in this as they had to create new modules for DNN3.x so they should be able to provide the main breaking changes that you need to look out for.
This is the main DNN asp.net forum:
http://forums.asp.net/90/ShowForum.aspxHope this gives you a starting point, thanks,