I have done this numerous times. Sometimes it's worth it and then there are other times it's too much work. Do keep in mind about backward compatibility, but this is pretty normal for most programs anyway. There are newer features in TS12 that TRS2004 and other versions never had. Bringing those things, with newer content back, can and probably will mess things up a lot. Besides, the map files and ground files are totally different format-wise than they were in the previous versions.
Keep in mind that SpeedTrees do work okay if you:
1) Replace them with better ones such as Pofig's offerings on his own website. They are not on the DLS, however. If you are looking for DLS available trees check out those by 2) Use McGuirel's wonderful trees instead of use both if you wish.
2) Thin them out. Yup. The old flipboard trees are bit closer and denser, making the Speed Tree replacement way too thick. This I found can really hurt the frame rates.
After that it's all up to you what you'll replace. There are some really nice track options now such as those by samplaire (username) His track begins with SAM. There are plenty of others as well others. The newer roads, such as the YARN series and the newer YARNISH roads are awesome. They're thick enough so that cars don't float. Remember that in '04 and '06!
John
