We just had a person come over and talk to us about window replacement. This person described their window replacement procedure as taking out the old window and somehow preserving the window frame rather than going down to the 2X4's. The sill is replaced with an adapter and the replacement window fits inside the frame hence is slightly smaller than the original window. This contrasts with methods of window replacement I've seen here and elsewhere. Can somone help me figure out if this is an acceptable approach? Thanks.