Boot Ubuntu mate on raspberry pi 3 b then enter “sudo rpi-update”, reboot and then it will work on b+!
Is this a speculation or have you tried this in practice? I have borrowed a RPi 3B and could try this if you are certain I would not be wasting my time!
I have only tried this only once on my raspberry pi. I doubt that this wouldn’t work but i would strongly recommend that you do not buy another pi if you do not want to because there are other ways.
Yes - I agree! I did the same and it worked on the RPi3B+. However, I then had problems installing INDI software for astronomy with the AstroPi3 script. After an hour of processing (downloading, installing, purging etc) I had to reboot and now booting stuck at the four raspberries screen. Happily I saved a backup before running the script.