You have multiple options.
Option #1. Get the new filter, and set it up with the new tank. Do a fishless cycle with pure ammonia. It can take up to 6~8 weeks without bacteria seeding. It can be much faster if you seed it with a piece of old filter media from your old filter, or if you transfer a lot of gravel over but it won't be as effective as the filter media transferring. You can also use Tetra SafeStart as bacteria seeding which was mentioned in this article
http://petskeepersguide.com/fishless-cyc...gen-cycle/
Once the fishless cycle is completed, you can move the fish over safely. You can transfer half the water over from old tank before move the fish over.
Option #2. Get the new filter, and have it run along side of your old filter in your old tank for minimal 3 weeks. It will gain bacteria required for the nitrogen cycle. But it is not guaranteed to have enough bacteria to take on all of your fish's ammonia once it is the only filter for your new tank. If you go this router, you can then use the new filter to set the new tank up before moving fish over.
Personally, I prefer option #1 for guaranteed safety reasons for the fish.
I have a question. Do you plan to set the new tank up in the same house as the old tank?
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