Agree with Dig. If a gun has a 2 1/2" chamber, the appropriate pressures are far more important than case and chamber length. Low pressure loads will fit nicely in 2 3/4" cases and eliminate the need to rechamber. Also, vintager loads are readily available for 16 gauge in case you don't reload. Further, note that recoil needs to be kept down via light shot charges at lower velocity.

Opening the chamber to 2 3/4" does not, in any way, make the gun suitable for modern "normal" shells.