I'm definitely not the LA expert around here, but it's my understanding that the 12 ft table can be converted to a 6 ft and the 10 ft can be converted to a 5 ft. However, changing your table back and forth in sizes probably isn't a good idea. For one thing, it's heavy and for another, if you don't get everything adjusted just right you run the risk of throwing everything, including your LA, out of whack.
I don't understand why you want to convert the table. I have a 10 ft table. I quilt up to that size and down to as little as a wall hanging. You can easily do crib size quilts on a 12 ft table. Just center it and only use that small area of the table. You're fortunate to have room for a 12 ft table. I've heard many ladies here say they'd love to have a LA, but just don't have the room for it.