Swapping blocks in an optional section

[mb324-unikent]

If you're allowed 0 or 1 blocks in a section, but you can choose between more than one type of block in that section, you should see a swap button.

This makes it clear to the user that they can change the block.

Instead at the moment you have to delete the block, and then replace it. I think this makes it look to users (well it did to me) that you can only delete the block, not replace it with something else. The risk is that you might not realise you can ever replace a block with something else, so you just leave it there.