Unless the chamber or forcing cones have been reemed out I I suspect 2 1/2 guns can handle proper 2 3/4 shells if the pressure is moderate. Problem is we need to anticipate the dumbest shooter in the world not the safest do the near universal use proper legenth loads admonition. That automaticity rules out 95% of the Gactory loads which most are too high pressure. By default dumbo buys shorter shells which almost all are safe pressure options.
In my doubles I just use low pressure loads in all of them. I cut down a thousand Federal paper cases at a time to 2 1/2 and only use low pressure loads in Federal. That way I know every Federal is safe for all my doubles. Two basic loads are 1150 FPS with an ounce and now a slightly heavier 1 1/16 ounce load which I very pleased with. A third slightly lighter load is 7/8 or 3/4 ounce for Skeet and Sporting clays. I dont feel under gunned and dont worry about wounded birds with clay targets. Reloading is such a gift to old double gun users.
