How to check for various blocks using a procedure

I'm trying to make a block convert into another block by checking if there is the same block underneath. I'm essentially trying to create border at the bottoms by telling the blocks to turn into a border if that block is not underneath it.

Thing is though, there are no OR statements for blocks where I can say "if the block below me is this block OR this block convert to this block," so my system just loops indefinitely between the border block and normal block because of the way my procedure is.

Anyway to overcome this?