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As for displacement leaks, do you mean leaks as in entity leaks or do you mean it's now "sewing" which gives you physical and visible holes.
For the entity leaks, displacements don't seal that map. You'll need to place a nodraw textured brush under the displacement to properly "seal" the map. Just be careful, that if a displacement dips into a nodraw brush, then you'll get a crappy looking black spot. You'll want to make sure that any dips you have in the displacement stay above the nodraw.
For the visible holes, you'll need to make sure the grid pattern that appears on the displacement line up with the side that you want to "sew" (or, make it so it doesn't have holes. It's like sewing 2 piece of fabric together). The easiest way to do this is make sure that both the displacements are either the same power (so, both power 2) or are multiples of each other (power 2 and a power 4). The better technique is to just make sure they are the same power. Then you need to make sure that the edges that you want to sew are the same length. So if you want to sew together 2 square displacements on a single edge, then the 2 edges need to be the same length and in line with each other. So both of their vertices are on top of each other.