I was wondering if its possible to just set back a barrel in 243Win a tiny amount to get a 6BR chamber?

Or is it more to it?

It should be possible, though the devil is in the details. It's a bit of a pain to pick up the existing tenon thread and extend it. It may be sufficient to extend the thread relief and the shoulder while using what's left of the existing thread.

If there's enough meat on the barrel it may be easier/better to just cut off the old tenon and go from there using a chamber reamer of choice. As a hobbyist it would be fun to do, but I would not pay a gunsmith his hourly rate to tinker with it.

The rifle I was looking at, is the Tikka T3x Varmint Stainless.. it comes in 243Win with 1-10 twist.

1.52 versus 2.05 in brass length gives you an idea of how much setback you'll need, plus cleaning up the chamber, etc...

Also if you want to use the bullets that make the 6BR shine at long range, you’ll need a 1x8 twist.