The best way to keep the beer is to go ahead and move them to your cornies.
By the way cornies make great secondaries!
How long they'll keep depends on a number of factors. Such as sanitation, exposure to O2 & light, plus the temp. the beer is stored at.
Good sanitation, reduced or no exposure O2 or light, keep the cornies cool and you should be alright for a long, long time.
Gas or no gas it really doesn't matter. But since you plan on storing them for a while I'd say hit them with a shot of gas then purge the pressure to push any O2 out. Many people hit & purge them a couple more times. I don't because I use the cornies as secondaries so they build up pressure on their on. I just pull the pressure relief valve a time or two inorder to make sure the cornie has sealed and to drive off any
O2 but whatever you're comfortable with is fine.
