I think the difference of opinion comes in because I (and I believe others) believe the 18-year-old isn’t the same person as the 55-year-old ‘you’ who underwent the procedure.
If the original body wasn’t destroyed which would be the ‘real you’? The original 55 year old you? The new 18 year old you? Both of them?
Both of them would be Oy!. Each would think of herself as “me.” Each would remember agreeing to the procedure and being anesthetized. Each would remember my whole life up to the exact point at which the 55 year old went under, even though only the 55 year old body actually had the experiences. The memories wouldn’t be any different for the 18 year old. But the 55 year old would have old age and probably poor health to look forward to. The 18 year old would have her entire adult life to look forward to. Frankly, this 55 year old isn’t that thrilled about her prospects without a construct, and not so enormously attached to living (this is particular to me, not to 55 year olds in general). As far as I personally am concerned, the kind thing to do would be to destroy the 55 year old before she regains consciousness, so that there never are two thinking, breathing **Oy!**s. Let the rejuvenated Oy! go on viewing it as a medical procedure she had done, that restored her to youth and health.
Admittedly, I would be significantly less enthusiastic if the entire thing were done while I was conscious, because, while I’m not particularly afraid of being dead, I am quite afraid of the experience of dying.
I’d actually want to have the memory of dying, if possible, for the new and improved me, just because I’d be very interested to see what dying is like. I think of it as a perk. 
We already sent the letter. It was returned. Sorry you had to find out this way.
Looks like pretty much all we disagree on is the importance of keeping the original then.
As for myself if the process was non-destructive of the original I’d happily make additional copies of myself, and as I said above I’d have each copy uploaded into a different type of body to compare and contrast (not sure why people are so attached to their current body apart from familiarity!). I’m not so narcissitic to imagine that the rest of the world would be thrilled with multiple versions of me running around though it would be fun and interesting.
If the process is destructive of the original I’d only use it as a last resort, if I was dying anyway, but I still don’t think the copy is anything other than another person who thinks he’s the same as the original. A different version of myself perhaps but still not actually the me that is me.