I recently started mapping platform edges for island platforms in my area and in the process, I sometimes want to refine the platform area - for instance, if the platform has an elevator or stairs to a passenger tunnel within. If the platform already is a multipolygon, this is no problem.
However, if the platform area is a closed way with area=yes and a member in PTv2 route(s) (role=platform), this becomes tricky. When I make the platform area a multipolygon in JOSM, its tags are properly transferred to the new multipolygon (good). The old area stays a platform member in all route relationships (bad). This means, that I need to select each route the platform is a part of, remove the area from it and replace it with the new relationship instead. This is quite cumbersome, especially if the platform is part of many routes.
Is there a way with JOSM to:
- make an area a multipolygon and
- replace the area in its relationships with the newly created multipolygon?
PS: I’m a JOSM newbie and, as for now, prefer iD as editor. So JOSM fans, its time to convince me that JOSM is the superior editor 
(to be honest, I hope there is a way in JOSM to do that. Fixing all those relations is tedious and error-prone…)