I mean think about it instead of messing with peoples voices this would have been a great oppertunity to reboot the franchise indroduce a new lead character. Sam fisher (micheal ironside) could still be the head of 4th echelon but more of a lambert role and then we have the new guy in the field who was trained by fisher so will have all of his moves and is younger and you could add new moves that he has developed himself.
Same for Grimm if Claudia Besso didn't want to do it indrouce another new character sam said he didn't trust her at the end of conviction perfect way for her to be written out.
Messing with the voices ruins the continuity its already hard enough with all the different versions of the same game but it is still possible to make sense, now however you expect me to believe that a man pushing 50 is as agile as a 20 year old and sounds completely different but is if fact the same person!!