I read this paperback novel in the early 80’s.
I clearly remember the start where an older, unfit detective (I think) is transported from Earth to another world.
On this world, as well as other people, there are repair type entities (nanobots?) that get to work on his body as soon as he arrives.
I remember memorable descriptions of his teeth being repaired and his mouth became full of his fillings which were pushed out to be replaced with shiny new teeth.
His body also changes, his beer-gut disappears, muscles grow, etc.
Later, weapons can be customised to fit the hand of the user perfectly (by the same nanobots). He tries one out that was modified to fit his companion, but, at first, isn't terribly impressed, until he realises his hand isn't exactly the same, obviously!
I seem to remember that it continued into some kind of grand space saga.