In what way? It is not guaranteed they'd be any happier in a tractor. They'd have growing grass to scratch in, but less space, which is a real tradeoff... Have you tried cutting a bunch of green stuff and shovelsful of dirt and such and putting it all int he run for them to scratch in? That can be a pretty good compromise.
Ideally, I will have the chickens in their permanent coop at night (good protection against the mountain lions, wolves/coytoes, etc.) and in the tractor during the day when we are home. If anyone has seen a tractor that big that is still easily movable, I would really appreciate your input!
Ah! Ok, what you want is a Lee or Salatin style tractor like would normally be used for meat birds. Or a cattle panel hoop style one, ditto. As long as your ground is pretty good 'n' flat and not real lumpy, you can have a tractor of that design up to 10x12' -- it will not be super predator proof and it will NOT be winterizeable, but if you already have a fixed coop and run those aren't necessarily big problems.
If you built a big but lightweight tractor on GOOD wheels you could roll it up to the run, open doors, shoo chx into tractor, shut door, then wheel tractor to wherever they will spend the day. A few cinderblocks would help temporarily anchor it against winds.
Not feasible for winter months, but then, you have a coop
Just a thought, good luck,
Pat