In "Our Man Bashir" it has to be pointed out that the heroes were ultimately stored aboard the holodeck, a system known to itself utilize transporter technology. Their physical bodies were clearly perpetuated by the holoentertainment system; it was made clear that only their "neural signatures" were stored as data, in a separate location.
Which was always a major "Huh?" anyway. Odo says "This is more complicated than just an ordinary transporter pattern", which is obvious bullshit. It's the transporter patterns of five people - what could possibly be more ordinary than that?
We already know that Federation computers can store small numbers of brain patterns, in whole or in part ("Return to Tomorrow", "Datalore", "Schitzoid Man" etc). Storing a body pattern along with the brain pattern might be a task a thousand or a million times more demanding (at least considering the number of particles involved), and cannot be done in practice - so the bodies must remain in the form of phased matter, be it in a transporter pattern buffer or in a holodeck's comparable systems.
As for rewriting the details of a person's pattern, I guess it can be done. But it won't give eternal youth the way you describe, because obviously the secret of eternal youth still categorically
eludes our heroes, regardless of the method. Either everything we think we know today is false or at best a partial and insufficient truth, or then the involved sciences in Trek developed differently.