Hello Chrisnic.
I've exactly the same problem as yours... I also made the BIG mistake to install the new version of FS Real Time, which comes together with the unregistered FSUIPC v3.50.
Before that, I was using FSUIPC v3.47 together with PMDG 747-400 without ANY PROBLEM.
Now, my 747-400 is totally useless...
And furthermore, FS9 crashes all the time when I'm loading the PMDG 737-800 as well !!!!
Two PMDG products corrupted for a single FSUIPC module change...!!!!
On the other side, Level-D 767-300 is still perfectly reliable since the very first day I've installed it.
But the idea to pay for two PMDG products which are always subject to crashes and/or malfunctions at each FSUIPC version change drives me CRAZY !!!!!