Hi,
One thing you could do is save a copy of the part and drawing at rev.level B. After making the copy set all of the features in part A to read only - then close and archive the part and drawing. Now you can make the changes you want to the rev.level B file. If anyone opens "A" by mistake, they will not be able to make modifications - unless the "read only" feature is disabled.
Hope this helps.
JW
P.S.
There is a check box: "copy associated drawings" when you start a new part - you would need to check this box for Pro/E to copy the drawing along with your part.