I'm sure there are some classes and I know there is some documentation. We've mostly been learning by trial and error, and help from the internet. The documentation is only the basics. Obviously there's so many different things you could do, it would be impossible to cover them all. I have to admit I get bored quickly reading the books too. I like to play around and just skim looking for what I need at the moment. I think what you're asking for is a combo box that looks up your existing parts. There's a special tool in 9.05 for that, but I don't think it's in 8.03. I don't remember how you would do it in 8.03. Sorry.
I do know that the upgrade from 8.03 to 9.05 didn't have much problems in the code. Any fields we added to the screens moved around and had to be repositioned, and of course the tables that changed caused problems. Mostly we had customizations concerning the GL accounts, and that was one of the biggest changes, along with the unit of measure. An upgrade tip would be to make sure your units of measure are standardized and correct before you upgrade. We have records that are wrong, and the only way to fix them is to set up a new part number, and our users don't want to do that because we would then have to change all the BOMs that used those parts, etc.
I hope that helps a little.