I know this has been mentioned and look at before but I want to add a simple solution if possible.
https://www.sweethome3d.com/support/forum/viewthread_thread,7442#56211
Puybaret: I had the idea to add an option to recalculate rooms like you proposed. But I already introduced a solution to simplify this operation: cut the room, create the room with double click and select Edit > Paste style menu item.
This works quite well but obviously does not copy the room name, font size or their location.
Perhaps you could write a macro that performs the above attached to a right click menu item 'Recalculate Room Area'
If you went further and added label and position copying that would be great.
This is a low priority feature request but I though I would get it added to source forge.
Windows 10 64-Bit
SweetHome3D: v7.0.2
Recalculate room points item was added to plan contextual menu.
You have to select a room and right click at a point where the room points should be recalculated to get this item. If the points don't need to be recalculated, the menu item won't appear.