This would be a good question to move to the frame builders sub-forum but I imagine it will be cutting out the old dropouts off cleaning up the area from paint and welding new dropouts on in a jig. It would probably not be a cost effective solution if the frame is a lower initial cost frame and if it is a nice handbuilt frame I would think that would really only be a situation you would need in a crash or some serious mangling of the bike and in that case I would reach out to the builder and see what they say.

I own 4 steel bikes and have had several others in the past and haven't once thought about replacing dropouts on any of them nor have had any issues with them. The only time I might consider replacing a dropout is to run different wheels or convert a frame to maybe a single speed or something or maybe if it is an integrated hanger and the hanger cannot be bent back or is broken.