My brain getting picked?
I don't think it's even possible to make an aircraft that's cross compatible without some major changes. There's always changes that have to be made in the code. It happened with the Saab. When we started making it, it was intended to be for XP9 and 10. Then 10 came, and we saw a few major changes happened. The weather radar being one of them. So we dumped XP9 development and went strictly for 10 only. It would have been way too much work with a very low ROI. XP9 was pretty much dead by the time we finished the Saab. So it wouldn't make sense to make it for both platforms.
If it's an aircraft with no plug ins, you can simply move the aircraft over from 1 platform to the next with no issues. If there are issues, opening the aircraft in the new version Planemaker, then saving it, usually fixes them. Some datarefs may be made inactive (which actually happened for the Saab in XP11), but it's not always a major problem. There's a new PT6 engine model in XP11, and that's thrown a bit of a spanner in the works for the Saab, so that's the thing we're working on now.
The other big change now is PBR in XP11.
Then to get the code right for the new version of X Plane...that could take a few weeks or a few months. It's kind of a shame that this happens, because it puts everything else on hold, but it's just part of the process and we have to get it done.