Can I use FSDT Live update BEFORE running P3D4.5 for 1st time?
It doesn't matter, and the Live Update will NOT do what you will need to do when running P3D 4.5 for the first time.
What you need to do after running P3D 4.5 for the first time, is to follow what GSX will tell you, that is running the
GSX Control Panel (which is NOT the Live Update), to disable default jetways which has been enabled by the simulator update. This is what GSX will tell you, and this is what you'll have to do. Nothing less, nothing more.
And yes, if you want to save a bit of time of exiting/re-entering the sim, you CAN run the
GSX Control Panel (again, NOT the Live Update), to disable default jetways, before running the updated P3D for the first time. But it won't make any difference if you do it before or after, the only difference is that you'll save the time required to start the sim, read the message from GSX telling you to exit the sim to run the
GSX Control Panel