Why put the fish through the extra stress? If you are planning on upgrading, then it would be best to wait on a fish with the larger requirements. What if something happens to you financially and when the time came to upgrade you couldn't? Then you would have to donate or rehome the fish adding even more stress than just moving tanks. I eventually want a panther grouper, but I am definately waiting for the larger tank even though I plan on getting him as a baby.
10 Gallon FOWLR (and macro-algae)
20 Gallon Low-tech planted FW
30 Gallon Reef