I've been shooting my M400 elite upper on my S&W 15 lower. I bought a PSA complete lower and put the M400 upper on it today. It shot well but I was shooting 2 inches high and one inch to the left. No big deal but I thought the zero would hold between lowers.

A few things could be happening here.

Theoretically, the POI should not shift because the mechanical elements that control the POI are in the upper receiver.

However, there is always the human element.

Things like trigger, and stock fit & placement can affect your accuracy. You also didn't say whether you were shooting with an optic or not.

Changing the lower is going to change the "feel" of the gun, particularly at the pistol grip, stock, and trigger. P/G and stock changes really change how the gun "relates" to you body. A different trigger can also create POI shifts as you pull on triggers of different weights/shapes differently.

I'm shooting a Vortex red dot and the trigger is heavier on the PSA.

Hmm...were you shooting from a bench? You could try again with a solid front/rear bag position-shoot a 5 rd group. Then, put the S&W lower back on and shoot another from the same stable position. Even though a red dot isn't supposed to have to be centered, make sure it is perfectly centered in the optic to also elliminate any chance of parralax. That will tell you something, could be the scope went wonky or the mount got loose or something about your position or maybe parralax was off with the PSA lower etc.

If it shoots back to POA again from a solid rest with the S&W lower and repeats the same shift again with the PSA lower, you know something is up concerning the lower. If it either matches up (it was a one off quirk) or the shift goes somewhere else, you know to look at the optic or mount.