An interesting historical note... M.W. Kelogg company people who explored this issue before most of the participants and readers of this forum were around specifically excluded "Repads" from their analysis and design standards. Why ??? beats me!

However putting a heavier pup piece of pipe with tapered butt weld ends into thinner wall pipe is usually a fairly easy thing to do. Also a big issue to be considered is just how attached is the repad to header????.... most calculations assume the pad and header acts as one solid piece but thay aren't.