Theoretically yes, but me personally I would be a little nervous about the two versions of LogicPro accessing the same preference files. Some people are saying it seems to work fine though. Make sure you compress your 10.4.8 LPX before updating so that you can roll back, but if you really want to be complete you'll also compress all the logic preferences and Application Support files too. or maybe it will work fine going back and forth. (shrug). let us know! but back it all up first.
1. Will do on compressing before update
2. will do on compressing preferences and application support files. (just to be safe and have a backup if needed)
I just opened a quick sampler preset in EXS24, using Mainstage. So yes...EXS24 can read the QSampler presets, and I presume Sample presets too. It must ignore the settings that are new and it doesn't know anything about. (knock on wood).
That is good news as well as I'm a heavy MainStage user and was curious about that as well. I can totally understand that MainStage would ignore andy of the "new" parameters. (At least till MainStage update comes out, which hopefully won't be too long after)
That being said, if you create a project in LogicPro 10.5 using instances of Sampler or QSampler, and then try to open the project in logicPro 10.4.8, the older version of LogicPro is going to complain that the Sampler/QSampler plugin doesn't exist. It doesn't know anything about it. You could manually put instances of EXS24 onto tracks and then load the Sampler/QSampler presets into them one by one...