I can convert a usual polygon into a multi-polygon quite easily with Ctrl-B. All tags of te polygon become tags of the new relation.
But how do I do the opposite? Currentyl I have a lot of relations that consist of a single polygon, and I want to remove the relation and have all tags moved to the polygon.
There’s a JOSM plugin called “relation toolbox” (reltoolbox) that lets you do this. You have to select a relation and then you can right-click on the relation in the relation toolbox panel and select “reconstruct multipolygon”.
It’s best to confine such changes to a small area unless all nearby mappers agree that it is needed. Otherwise the next mapper is likely to come along and convert all adjoining polygons back to multipolygons.