I've done something similar to loko, but I was only moving the fish across the house so they weren't in their bin very long--I gave them an air pump to be nice.
The intermediate container idea is generally the way to go, but with that CO2 thing...if you're only going with a short time in the intermediate container (a couple hours?) maybe it'd work out.
I'm not sure about that CO2 issue though--can you turn it off for a few days and slowly get it going again? I think what I'd try to do is turn off the gas, drain the new tank so that it is only partway full and add as much of the old water as possible--with luck getting enough so that it's no more than half new tank water and still enough total water for the fish to live in. Move fish (who will be happy since you just removed their tank water), give them a day or so, fill tank the rest of the way, give them a day, turn on the gas....but I don't know how much that would mess up the tank you just got all set up, or how much water you have available form the old tanks--probably not enough to fill a big chunk of a 150g...
Are your fish fragile? Intermediate container is probably the most practical route.