Yeah, what he says, and then reinstall PMDG et-al addons manually. Once verified New one is working then you can delete any and all things off old one concerning this sim, or as alternative thought.
If you are keeping old machine, nice place for all that stuff to sit as backup if necessary for 6 months to make sure you did not forget anything which needs to be brought over. And maybe could use it for data storage too. Lots of options on old PC. Movie-stream to TV, store music files, pipe through stereo, just world of things to use it for.