I could installed 1.15 on top of 1.14 at least twice. So I can confirm, that this works, but as you already found out, it does not work reliably. It took me around 1 to 5 retries to get a new condi firmware installed, no matter what version it was. If I recall it correctly the installer app did not crash but just reported that the upgrade failed after some time. Then I could just restart.