As I wrote in bug #275682 (too late probably), the new ebuild changes the udev rules order. The old ebuild installs 70-hpmud and 71-hpmud_support. The new ebuild installs 70-hplip and 56-hpmud_support. I'm not sure about the consequences, but I think it is good to retain the rules order.
Created attachment 206166 [details] hplip-3.9.8-r1.ebuild that fixes the order of udev rules
I think the renaming of the udev rules is no longer necessary because the responsible rules file 65-permissions.rules does not exist anymore, thus I have removed this part of the ebuild completely in hplip-3.9.8-r2. It looks as if newer udev versions are setting the permissions to user lp and group lp anyway, so nothing gets overwritten.
Works for me, thanks! Feel free to close the bug.
Closing. If the rules still need renaming for other reasons, we will rename all of them as suggested.