2 ways come to mind, both quick , cheap, dirty.
Add straight 2X4 sisters, offset by whatever is needed to make each stud look even after the drywall is hung.
Or rip 2X4 to the thickness needed to shim out each stud, and face nail to the studs individually, to bring them all even for drywall.
Then of course, support the upper with temporary wall, and tear out the old and replace it, but you say that is your last desire to do.
ED
Add straight 2X4 sisters, offset by whatever is needed to make each stud look even after the drywall is hung.
Or rip 2X4 to the thickness needed to shim out each stud, and face nail to the studs individually, to bring them all even for drywall.
Then of course, support the upper with temporary wall, and tear out the old and replace it, but you say that is your last desire to do.
ED