Nope, if you add a rating to the underlying licence you go through the validation process again to add it to the 61.75. It's the "may be placed on the certificate" bit as you point out that applies. The 61.75 shows the ratings you are entitled to exercise.
XII on the back shows you what privileges you may excercise. There is no automatic addition.
To add the ratings the UK CAA have to release your details again and the FAA have to reissue the certificate.
In the days when I had a 61.75 it was exactly what I had to do in order to add MEP. I converterd to a full CPL/IR as a result of the faff.