JetRanger, I think you are probably over-estimating the complexity of the situation.
Instead, you should see it exactly as the day FSX appeared on the market.
The thing is, there are not so many sims to choose.
- FS9 is obsolete, but people who still use obsolete computers might want to keep it, I don't know. Anyways, it's not going to create any impact on the modern sims.
- P3Dv1 and P3Dv2 are totally obsolete and can be forgotten for ever. It's like they never existed.
- P3Dv3 is also kind of obsolete, excepted if you want to continue using some old addons (FS9 planes for example, which are rendered superbly in P3Dv3)
- P3Dv4 is the current sim of choice.
- FSX... nobody needs that anymore. All of the important addons have been modified to work with P3D already, so there no reason at all to keep FSX on the disk. Unless you are really enjoying the trouble, the installation problems, the activation problems etc....
- FSX Steam is obsolete but it's ont Steam, so it's easy to install/uninstall anyways. The question is: why would you want to do that ?

So that really leaves three choices only:
- P3Dv4, if you want to benefit from the new technology and the new addons
- P3Dv3, if you want to keep enjoying your current addons
- XP11 if you want to try something totally different.
The "upgrades" that seem to worry you are not really a problem. The addon makers have just released some new version of the installers for some of their products.
If you bought for example some OrbX sceneries, the OrbX tool will handle the conversion for you.
If you bought some planes from Careado, A2A etc.... then you should have received some emails and you simply need to redownload them, as if you just bought them.
If the addon maker charges anything, then you have to buy again.
The same happened when we switched from FS9 to FSX really. Nothing to worry about
