Let's say I'm going to live until 50 (head+body), but then get a replacement 20 year old body when I'm 40. What happens to me? Can I make it to a higher age now

Let's say I'm going to live until 50 (head+body), but then get a replacement 20 year old body when I'm 40. What happens to me? Can I make it to a higher age now? Will my mind eventually just turn into mush anyway?