Thank you Umberto, but if I use the current version that might mess up my FSX.
The current version surely won't "mess" your FSX. But that's irrelevant, since installing into P3D it's an entirely separate install.
I guess I have no choice?
You obviously have a choice, you can choose between:
1) installing the current version under P3D and leaving FSX with the version you already have there
2) installing the current version under P3D AND FSX. Which means, running the installer twice, once for each sim.
The only CORRECT choice is of course #2, not because #1 will cause "problems", but only because there's no reason why you would want to continue to use an outdated version in FSX.