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.