Yes Adam, you can change week by week. The same way you could change your broadband or mobile phone provider every week. It’d be stupid, but no-one’s stopping you from doing it.
The two companies you chose are multi nationals, the clue is in the euro figures. Not so relevant for a discussion of the UK energy market.
I’ll ask you this then, what is an acceptable profit for an energy company to make? Remember that tax is paid on these profits and in the course of their daily business these companies directly and indirectly support tens of thousands of jobs. No profit = no investment = no jobs.