I think there will still be a lot that can be mined going forward and back. We're using spell lists from RMSS and even RM2 in addition to RMU lists, for example, and you could use RMU lists in the previous editions if you wanted to take advantage of them filling in the blanks. There are some specific types of lists which will not translate as easily (e.g. healing is different, things that affect the structure of the combat round like haste) but the majority will be fine. The presence of tools for creating races, cultures, and professions means that if you do move to RMU, you are not left on your own if you need to convert things from previous editions. Creature stats will be different but that's why the entire creature list is being converted -- just use the new ones -- and again there is a system for creating creatures if you do need to make your own. Character skill bonuses are going to change, but there are already big differences in those bonuses between RMSS and RM2/RMC, because of differences in how profession bonuses work (flat in RMSS, level based in RM2/RMC) and higher numbers of ranks in RMSS. I've also seen a fair amount of discussion about converting modules from, say, Pathfinder, and converting between RM editions is going to be way easier than that.
There are other changes like the structure of the combat round but I don't see that as difficult to house rule. Of course the more tweaking you did, the more there is to adapt, and RM2 with all the companions is more of a kit for assembling your own RPG than a fully actualized RPG....