It should take a simmilar ammount of time to; one is delete->redraw, the other is (move) redraw->replace. You'll need the utills plugin to do it. Conflation works very well when you need to replace geometry en masse, because you draw geometry on a new data layer meaning that you can temporarily hide the osm data layer allowing you to view the imagery directly. You'll need the conflation plugin to do this, I recommend reading its about pages linked to when installing the plugin and https://wiki.openstreetmap.org/wiki/JOSM/Plugins/Conflation , then opening two new layers in JOSM so that you can freely experiment and play to learn how to use it without worry. It talks about using an external database/source; you'll essencially be creating that footprint database before conflating.