It's not fair on the fish to cram it into such a small space - tangs need swimming and grazing space.
If you move to the 55gal, it still won't be big enough. And, when you uproot the fish and put it in another tank, it's simply not fair.
Choose another fish - I had to come to terms with this a few weeks ago, after having my heart set on a Regal Tang, my 40gallon simply isn't big enough. You reach a stage in the hobby where you gain a lot of respect for the animals and their environment, more-so than just getting a pretty-looking fish!
Don't do it