Get it evaluated by a good smith like Merrington or Kearcher to determine if you need to actually do anything other than fire loads that do not exceed the pressures for 2.5 inch shells.
Depending on how much wall thickness remains you may not have an issue.
If it is determined there is not an issue then go ahead and ensure the case clearly note 2.5 inch low pressure and consider stamping it on the gun and go on and shoot low pressure 2.75 through it without much worry.
Then again if the evaluation comes back the walls are now dangerously then either of those two listed above could further drill it out and put the correct sleeve in.
