What to consider when upgrading Mods?

[biocyberman]

Hi there,
Many times I want to use some modifications (mods) which were written for older versions of vbulletin. Problem is that the authors of these mod maybe too busy to upgrade their mod by themselves. So I want to install, test, debug and write additional code if necessary so that these mods can run well in current version of vbulletin, i.e vb 3.7.3. PL1 or vb 3.8. I guess those mods were written for vb before 3.6 will require more work. Nonetheless I would like to know what need to be considered when trying upgrading these mods. I am asking because I have been working with vbulletin for a while but my understanding about architecture differences of vbulletin versions is very limited. Welcome any comments suggestions and guidance