To start a player with a higher future rating but lower current at a position over a player with a higher current and lower future?

Depends on the players individual attribute ratings. If you are switching a player's position, his overall will drop, but his individual attributes will stay the same. Similarly, your rookie LT might have a lower overall than a vet, but his pass block actual may still be better. Simple answer: it depends.

Lets use this as an example. Veteran LT has attributes of 60 current / 75 future. Drafted rookie RT being switched to LT is 43/83. Is it better to have the vet start because he is at 80% of future or do I throw the rookie who is at 52% of future into the fire knowing he will grow?

Post the two players.

I usually start the young guy unless you notice he's struggling. Oline I find the eye test is more telling than the ratings. I have some good rated lineman who give up tons of sacks. I usually use preseason to tell if certain lineman will be good. If his pass block is really low on current he might struggle though, just platoon them and see who is performing better

I can't do that because it's hypothetical

Depends on a few things.

Can you afford to, in the Win Loss Column, start the Younger Guy? How much of a drop off in Talent is there? How good is the Vet?

Say the Vet is better on Current, but still just meh... but the Rookie can develop from **** to Great. Then I put him in, get that Positional Experience building and don't look back.

But if I have an All Pro Vet, he's playing. Rookie needs to sit and develop, get Pre-Season Snaps, rotate as depth, things like that.