I have done a simple check and it seems both www.dyndns.com and freedns.afraid.org give 1 min TTLs. It's strange you can't change that value (a way of avoiding too much load?) I guess we could workaround it by cnameing to some of their domains several hours before the move though it seems a bit egoistic. Just as a reference, i prefer the later system as it doesn't have profit, much on the good free way.