Allow changing kits for multiple projects

Use case: I have around 100 projects open, and want to migrate them from a kit using 5.12.x to 5.13. I install 5.13 side-by-side (still using 5.12 if I fix bugs in an old project branch), and get a new auto-detected kit.

How do I change my ~100 projects to use this new kit?

Currently, my options are to do it by hand (very tedious and time-consuming), or write a script to change the .pro.user files (not so easy, and error-prone).

It would be very helpful to be able to multi-select projects and do operations like "add kit", "remove kit" (given that all projects use the same kit) and "change default kit"