We have been using only P3D 4.4 for months now and, of course, everything already works out of the box, with no upgrade required, since we install everything in the proper way (using the add-on.xml file, everything outside the simulator), and since 4.0 came out, we are no longer bound by the precise release build.
So, in relationship with the upcoming upgrade, you can do the cleanest and safest update method ( uninstall the sim, reinstall using the full installer ), and it won't affect any of our products installation.
The ONLY thing you'll have to do, if you have GSX Level 2 installed, is that you'll have to disable the default jetways again using the GSX Control Panel, because updating the sim would restore the default jetways.
This covers the most basic update questions. We'll follow up this post with a subsequent statement, explaining what this new version is really about, clarifying the PBR issue (which seems to be required, reading the first user comments), and what does it means for FSDT products, present and future.