You guys may be talking about to separate things.
1) No psyker that I know of may only switch out "some of" their powers. They always switch out ALL their powers for BRB powers, or none. However, according to the main book, you may choose switch out your powers, or not, each game, and if you had more than 1 power, could even use multiple disciplines. Moreover, you can choose what table to roll on for subsequent powers after rolling your first power....i.e., pick your first power from telepathy, see what you get, and based upon that choose whether to do Telekinesis or Biomancy next, etc.
2) Sam has decreed that for tournaments, all powers must be picked in advance (whether to switch them out, and for what disciplines) in advance, for the whole day. This is so as not to "tailor for your oppoent", and is similar to how they do it fantasy.
Personally, I'm wicked against that, actually, as all the disciplines, aside from divination, are horribly underpowered (oh, look, remember I wrote a post about it?
http://prometheusatwar.com/2012/08/the-problem-with-psychic-disciplines-and-rulebook-powers-in-6th-edition-40k-2/) there's not much problem with people "tailoring their powers"; they're generally paying too much for too little, anyway.
Is there a way to campaign against this artificial restriction, either with Sam or Chase? It's not much skin off my back, I'm likely to take divination 95% of the time, and otherwise it would be codex powers I had a specific plan for. But generally speaking I think people without access to divination should be able to tailor their powers to their opponents....it's one of the few things they have going for them.