1. Create a new project configured with just one kit.
2. Build it.
3. Go to Projects mode, Build settings.
4. Click the small arrow at the kit's tab and select an incompatible kit from the "Change Kit" submenu.
5. Try to run the project.
   It will crash right away because it's still built with the other kit.
6. Select "Build" -> "Rebuild all" from the menu.
7. Try to run the project.
   It runs.

What are the odds that a built project can still be run with a different kit? I think changing the kit should mark the project for a clean rebuild. Creator might also ask the user about it.