Yeah a Koran is best suited in a larger system. Not to say he cant be put in 75 for a short period if you pick one up as a juvenile. I have a Quenn in my 72 and she seems fine, has never destroyed any of my corals. May nip here and there but I dont have SPS or clams either, just softies like leather, shrooms, and xenia.
I'm also upgrading to a 210 next month (cant wait!). I already have all my equipment ordered so its only a matter of time...
My Quenn has been in my 72 for about 6-7-8 months (not sure) but she is starting to change and the 210 will be more than enough for her. My main concern is stunting her growth and causing stress on the fish, which is why I planned ahead to have a home set up by the time she is ready for it.
To answer your question, its not impossible, but just get the wheels turning on a larger system before making the purchase. For instance I already had my tank and skimmers purchased when I bought my Quenn. A short period should be okay for a Juve Koran