acct-user/mythtv modifies the existing user if that user is not logged 
in. Mythtv has been around for years and users have created specialized 
configurations for the mythtv user. They do not like changes to the 
mythtv user when upgrading.

A couple of solutions:

1)
the ebuild does not depend on acct-user/mythtv when the user already 
exists. This prevents both the changing and deletion of the existing user.

What is the gentoo way to discover a preexisting user? Repoman does not 
like anything I tried using egetent passwd mythtv

2)  Add something to the acct-user/mythtv to (conditionally) prevent any 
change if there is a preexisting user on the system. The acct-user 
unmerge would also check that the user was not modified since it was 
created by acct-user. This seems like a lot of work.