Hey cichlid34,
I have had to seal a 10G aquarium (over 20 years old I borrowed from my brother) three times. I used a rule of thumb of 78 hours to allow the aquarium silicone to fully cure. Not to hijack you thread, but I feel the need to explain this. I sealed the tank initially (1). I set the tank on top of my mini refrigerator. When the refrigerator "motor" thing kicked in the fridge would rattle and shake. IMO this is what caused my awsome seal job seams to come undone. Reseal number (2). Like the genius that I am, I set the 10 right back up on the refrigerator again! Reseal number (3). I have had no problems with the tank for 8 months since setting the tank on a solid stationary surface.
I think that you should be OK as long as you transfer the subtrate and filter from your cycled tank to the sealed tank.
A couple MAJOR area of concern that I think that you should consider though:
1. The sealed tank is twice the size of your cycled 10G. Please make sure that the filter that you are transferring to the sealed 20G tank is a MINIMUM flow rate of 100 gallons per hour. The 10 could have been adequately filtered at 50 gallons per hour, which will not suffice for your 20G tank.
2. The profile of your 20G is considerably different from your 10G. If your 20G tank is a 20 long then the profile difference is even worse. Please be prepared to add additional subtrate if required. (This is only my oppinion and backed with NO fact, so more experienced fish keepers please chime in so I am not givin a fellow fishkeeper bad advise.) IMO if you need to add new subtrate in addition the the transferred subtrate, I think that the new subtrate (WITHOUT the benneficial bacteria) should be placed on the bottom with the transferred subtrate (WITH benneficial bacteria) resting on the top layer.
PS. They sell subtrate containing benneficial bacteria at LFS and even Petsmart. It is however unbelievably expensive ~ $21 for a 20 pound bag! Again I am not experienced enough to be able to tell you how effective these subtrates containing benneficial bacteria really are!