There was a relatively
similar thread a while ago.
In that thread, the situation was slightly different, as the player waited for Round
1 pairings to decide which deck he wants to play.
However, the general opinion was that it is okay to do so, even though it might not be exceptionally sporting.
The situation you described is very similar. For tournaments without a decklist, a player does not have to decide on a particular deck before they start playing, but once they have used a deck, it must be returned to its original composition before each subsequent match.
As for a particular rule I could point you to, there's no explicit rule that restricts when players have to decide which deck they want to play.
However, if you think that behaviour is causing problems (perhaps because other players are feeling uncomfortable about it), you can always talk to the player and express your concerns. In most cases, that should change the behaviour.
Edited Sebastian Stückl (Nov. 12, 2014 10:31:57 AM)