As part of switching Gnus group names to fully-decoded strings, I left
some code (really horrible code) in place to make sure that the group
names stayed in their previous encoding in Gnus' save files, so that
people running Emacs from git builds could switch back and forth without
worrying about the save files becoming unreadable. Right now, there are
several routines in place that decode non-ascii group names on file
read, and re-encode them on file write. This only has an effect on
non-ascii group names.

Now Emacs 27.1 is on its way, and I would really like to make this
change permanent. The attached patch moves all the ugly compatibility
code into an upgrade routine as part of `gnus-convert-old-newsrc'. Users
will be prompted once to upgrade, and then the various files will be
fixed to use properly encoded group names, and you won't be able to go
back.

Again, this only affects users who had non-ascii group names to begin with.