Indeed... generally you should not have to change a Perfect Match because it's supposed to be based on a reviewed previous translation.  If it was used to just get back to where you were before then the option to keep the statuses and match values as they were should have been used.

It feels a little as though you are having to fix poorly prepared files from someone else.

While I understand that in principle Perfect Matches should never be changed, in real life they quite often do need adaptation, and search-and-replace can be time consuming if you need to make changes to various different PM in one file. 

I have found that in such cases the Integrated Segment Actions app (https://appstore.sdl.com/language/app/integrated-segment-actions/530/) can provide a solution. Simply use this app to change the Perfect Matches to unlocked 100% matched TM matches and when you (re)open the file any changes you make to PM will be auto-propagated as normal