It sounds like it would be an irresponsible move on your part to take on the care of an animal you don't really want just to get another tank? Beardies need their space and chances are the animal needs the tank he's in and not something smaller just because you want the tank for something else.
Beardies require considerably more effort on a daily basis to care for than a snake. Please consider carefully before taking on such a responsibility for an animal you aren't passionate about.
Plastic tubs are a great, inexpensive way to house snakes if that's all you want out of this deal.