I would make the internal transition at least 45 degree angle as a square corner will break down quicker and catch cleaning patches. Like you said, it would be modified but hardly noticeable.
 
Set up and zeroed in on a lathe a 5/16 reamer would give you .3215 with a 45 degree lead and an 8mm reamer would give you .315 with the same lead angle. Done instead with a single point boring tool would give 100% concentricity to the existing rifling. Reamers might want to follow the existing erosion/wear unless cleaned up with a boring bar first. Your gunsmith will know all this already.