If by low 20s you mean they're 20 gallon longs, I would think it would be a little small for a 4-5 year old BP, but it really depends on how big your snake is. The rule of thumb I've always been told was that your enclosure should be big enough that your snake can stretch out along two sides. For example, if your snake is say 3 feet long, ideally you'd want something that is three feet across the front and down on side, so on and so forth. I know people think BPs are pet rocks, but I regularly peak in on mine at night, and I'm always shocked by the amount of exploring he does when the lights are off, so a smaller enclosure just wouldn't do.
As far as your question about moving. In my experience, I've had no issues moving drained fish tanks that still have plants and substrate in them. Now obviously all the water is out, but as far as having a little weight in the bottom, if you have someone to help you I think you'll be okay. It really again depends on how much substrate we're talking and how heavy those tanks are. A empty 75 gallon is a task to move around, so I would really be careful about trying to move it with a bunch of dirt and rocks in it.
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