Swapping top ends of PPs and PPKs depends on when they were made. If they are both from the same period, it shouldn't be a problem. Slight manufacturing variances over time can make swapping a problem, but theoretically they should swap easily enough. Removing the barrel can be a problem. The barrels on both are pinned, and if they're old, they may be really stuck. Unless you have some experience in doing work like that, it might be best to have a true gunsmith do the work.