Someone closer may chime in that has the experience to change a barrel. If not, I would reach out to Got Uzi, above, in Uhrichsville, Ohio. He does excellent work and his prices are very reasonable. If your not interested in shipping, Google your location to Urichsville, Ohio and see how long a drive you have. Send Got Uzi or John a Private Message with your contact information and I am sure he will reply. Just click on his name and you will find the Message button.
What type of Thompson do you have? Is the reason for a barrel change a bulge?