Everything you change or modify will reduce collector value.
First off, an M-28 is not exactly a collector gun, unless it is in excellent/exceptional condition. An M-27 is a different proposition.
Second, never say always, never say never.
Once upon a time (3-4 years ago) I bought a used 4" S&W M25-5 (45LC). That is a ~$750 revolver.
Since it had been worked over by Hamilton Bowen, the asking price was $995. I paid it without blinking, and it was the sweetest revolver I have ever seen, held or fired.
It was not perfect, but it was too nice to carry. I don't have enough money to own "safe queens"--if I don't carry it or shoot it regularly, I sell it and buy something else.
I sold it about two years ago, and when I put it up for sale at the same price I paid, it took only a couple days. The first guy who looked at it paid my price without blinking.
Bottom line--Modifications will not always diminish an S&W revolver's value.