You can leave FSX as it is, Ahmet. You may need to install some stuff again in P3D, other things can be copied over. The folks running it can surely help with that step.

I'd keep FSX as a backup, even if you like P3D. Some stuff doesn't run in P3D and most copy over thingies aren't supported. So whenever there is something not working, you have to ask yourself if the product is faulty or if the other sim causes the problem. With running FSX, you can test that.

Be aware that some payware devs don't support P3D, so support threads with that sim may not last that long or just won't receive an answer. Some seem to get very upset on licensing issues.

Let us know how P3D works out for you. I wasn't impressed so far and reinstalling things or the sim itself (with every update) is not what I'm looking for. If it had turned out to be much faster, that statement would look different of course.
If version 1.4 generally unlocked it, let us know. The video above doesn't look like it did with the same settings. But maybe there's a trick somewhere.