So they will not be store the transaction in memory / broadcast (it to other nodes) before XX Specified Block?

My impression from playing around with it in on testnet, was that the QT client default behaviour was to store a lock_time transaction in memory until said block, but would not accept a higher seq. num replacement for it.

I guess what I'm really asking is along the lines of something like this: suppose someone creates a Lock_time transaction open until a certain XX block, now if a certain event doesn't happen before said XX block, they want to cancel the transaction by broadcasting a replacement set to close at an earlier block (and suppose there is someone willing to accept the higher sequence transaction and mine it) is there a chance that a major pool will in fact receive the first (lower sequence version) of the transaction, store it in memory until said XX block but not accept the higher sequence replacement?