I'm assuming the new tank is going in the same spot that the old one currently occupies? And that you have been doing proper regular water changes on that old tank?
If this were me, I would put the bichir into a bucket of water from his current tank, cover the bucket, drain and remove the old tank, set up and fill the new tank with tap water of the correct temperature (easy to do), place the old cycled filters into the new tank, move the old heater to the new tank without touching the controls, treat the water with dechlorinator if I used tap water, and then gently pour or, better yet, net the bichir into the new tank. Simply put, it's a 100% water change. No acclimation or other nonsense required. Acclimation is possibly useful for fish that are going long distances, and from one set of water parameters to a vastly different one. That's not the case here.
No bagging required, certainly no schlepping the fish to a LFS for them to bag, no fuss, no muss. Maybe leave the lights off until the next day, and relax.
Relaxing is the most important part; this is not complicated unless you make it so.
Edited to add: Aaaaand...once again, someone (
Rocksor
in this case) types faster and more concisely than I did...
Edited again: I agree, those laser temperature guns are worth their weight in gold.