We have lots of presets for it already so I'm hoping I don't have to purchase Lightroom 5 just to use the D750. Does Adobe drop all support for Lightroom once a new version comes out, or is there a chance they might come out with an update for the D750 since it is semi-professional?
Side-note: LOVE THE D750! Waiting for this camera forever. Used both the D610 and D800 to try and replace my D700, but they just couldn't cut it. The D750 is king. (or at least prince to D4)
At the same time, they seem to be trying to make updates redundant by persuading us to go cloud base
You don't have to purchase Lightroom 5 you can get an upgrade for half the price
but LR 6 might be round the corner
I don't even see the D750 on there yet, but the D810 is minimum LR 5.6. So I'm guessing that yeah, you are going to need to upgrade LR. I think there is a procedure for importing your old presets though, so I don't think that should be a problem. This is all from a LR 4.4 user, so take it for what its worth.
It's possible that an update to LR4 is coming, but with the investment you've made to the D750, you might want to step up to a later version of LR - that's what I would recommend.
Presets can be updated as dissent mentioned.