The answer to both questions is yes. It's possible but a lot depends on the design of the wall mount and TV. I've had some success reusing wall mounts in some cases but not others. It's difficult to tell until the TV is in hand.

 

I've used the old (existing) wall mounts adding a small piece of plywood, basically mounting a piece of plywood with wood screws to an existing wall mount. Then using previously drilled holes in the plywood that aligned with the proper sized / pattern vesa mount holes on the back of the new tv. Using slightly longer bolts ( from hardware store) to compensate for the plywood thickness,attached the tv to the plywood basically. As long as the plywood attached to the tv is only slightly larger than the vesa pattern I am not worried about the weight of it or a larger tv being used on a smaller rated mount. Because, the newer tv sets are now lighter than the older sets, but easily verified by looking at specs I guess.
Like you I do not want to remove old mounts and make more holes in my wall, besides you never see the plywood. You could if wanted also use extra bolts to secure the tv to the plywood if the tv has more than one size vesa mounting hole patterns on it ? Maybe I am wrong in doing this but it had worked for me in the past.

 

It's probably a little late to suggest this but adapter plates are available that will let TVs be used with mismatched wall mounts. Most wall mounts will accommodate a variety of VESA sizes on the TV so they are not usually needed.