Hey man, what sort of biological filtration are you using in your main tank? The reason why I ask is because if you are using a wet/dry, you could remove some of the bio balls, or whatever media you have in there, and use it in your QT tank to kick start the filtration. You can purchase a small canister filter and put the bio balls in it. This way you still have some bio filtration going on.
If that is not possible, then yes, you will need to be prepared to change a lot of water, which may stress out your fish. How many fish do you have and what size are they? If you have really small ones, and you don't overfeed, you may not have a major problem.
Here is one last suggestion. Purchase some live rock, the cheapest you can find. Mix up a bucket of salt water and scrub the heck out of the rock in the salt water. Once you have scrubbed away as much algae and other life as possible, put it in your QT. That will act as your bio filter. Yes, you are going to have a lot of the micro fauna die on it, but the actual bacteria will be fine. You can expect the ammonia levels to go up as you drop your SG, but the bacteria will catch up. Sacrificing the hidden life on a piece of live rock is better than killing your fish. Some people may claim this won’t work, but the bacteria that in live in live rock are exactly the same as those that live in your filtration system. Neither will die under lower SG. Some of the inverts and other critters on the live rock will die, but again it beats having your fish die. Just my opinion.
Good luck man.