Reincarnation isn't about stealing another person's body and therefore ousting their soul in the process.
It is about being reborn - given a second chance, a next life - however you want to word it. It is more of a continuity thing.
Taking over another persons body tends to come under the ghost/possession genre in my opinion.
As we are dealing with fiction, I don't see why any one persons religious beliefs should take issue with the idea of reincarnation.
After all, we are a society of unique individuals who all make our own choices in life (after life if you do believe in reincarnation) and respect other religions, faiths and beliefs around us.
So I am not sure I understand if you are asking for a Clark who has been re-incarnated - ie born as a baby and lived another life as a mortal human who may or may not come across his 'soulmate' - a re-incarnated Lois, or if you are asking for a soul-replacement/possession fic where Clarks "soul" is intentionally or otherwise placed into another mans body (which is NOT reincarnation in my opinion) and meets the L&C Lois.
To answer the 'why hasn't Clark has his soul switched' question - I would say that if you look at the TV show it has been written at least once - S4 "I've got you under my skin" (and yes - I cringe at that storyline... but it was IN the show), or even the more popular "Soul Mates". However, you also have to think about the fact that most fics in L&C deal with Superman as part of the equation, and without Clark/Superman, part of the story is lost to this reader base who are here because of the foundations laid by the TV show. Lois is mortal. Clark is not exactly mortal. Unless you re-write the show, you can't change that essential and basic foundation.
Perhaps there is scope for a role reversal, where Ultrawoman remains the constant and has to wait for Clark to be re-incarnated?
Again though, I can't say I am entirely clear on what you are expecting from Mr Jones, given this is a forum where people write about Lois Lane, Clark Kent and Superman...
