A conversion may not be as easy as it first appears. The entire hose/regulator assembly will sit lower. It may cause problems with the main hose being too long, and you may want to cut the hold down all thread to the shorter length to make it easier to deal with the shorter tanks.. If these mods are done, then switching back to the 30s would be more difficult.
Costco sometimes has the 30s really cheap.
If not, get them recertified. The one time you need more capacity, you will be glad you kept the larger tanks. And at selling time, the 20s would be a negative for some potential buyers.