How often are you going to need to move it? Daily? Between uses? Weekly? Monthly? Frequency will dictate how sophisticated you get, here.
How heavy is your bench? What's it made of?
You might could get away with just a couple simple hinged flaps with casters on 'em and a short "catch" that raises things up 1/2" or so on the casters.
You might could get all sophisticated, too. First thought I had was a scissor jack on each end with the casters bolted to it. The trouble you're gonna have, here, is that scissor jacks aren't terribly stable in motion and you'd probably have to rig up some kind of "stay puttedness" so the forces of motion aren't born by the jacks.
Another way is to copy a thing I saw on NYW. The casters were on a hinged platform on each end. They just sit on the floor, angled up a little bit - they're not fully engaged or bearing the weight, yet. When it came time to move it, ya lift the end of the bench up maybe 2" tops and the casters, by way of gravity, fall down to fully extend their range of motion - at the same time, two hinged "flaps" fall and lock the platform in place. You set the end down and the casters are now bearing the weight of that side. Repeat on the other end. The flaps have twine tied to them so that you can pull 'em up outta the way to set everything back down again. I thought this was a nifty mechanism, and very simple. A couple hinges and some string and yer good.
If your bench weighs as much as ten cows, then maybe it isn't so easy to lift - but that can always be solved with a reversabl Bessey clamp in "spread" mode. I lift all sorts of heavy things up a few inches with that method. It's a handy way to raise things enough to get moving dollies under stuff, too. If ya have it handy, a floor jack and a post works well, too.