Will taking my Reba 29er forks up from 80mm to 100mmm make the bottom bracket higher?

Yes, but I don’t think you will notice that much difference unless maybe you are doing silly steep technical play biking?, then you might feel a little more perched on the bike rather than sat in it. But I think that will also depend on where you are starting from in the first place. If the BB is already high, then you will probably notice it being a bit higher. If it’s a low BB, then maybe you won’t feel much effect from a few more mm higher.

If you run more sag it will help a bit, but be wary of the frame warranty as sometimes it is limited to a specific length.

once sag is taken into account you’ll only be looking at a raise in BB height of somewhere around 5mm