Sometimes the situation occurs when you start to switch Power Companies and then realize that you have made a terrible mistake. It's okay, don't panic. Call your old Power Company and tell them that you do not want to switch. You have about 2-3 weeks to do this. If you call during this time your old Power Company can stop the switch and everything stays the same.
After 3 weeks you generally have to switch back, a process that takes another 2 weeks.
EXCEPTION - If for any reason any Power Company switches you without your consent your own Power Company can change this. This can happen for a number of reasons, the sales agent is perhaps less than honest, or your neighbors have accidentally signed up for electricity for your property, etc. Your Power Company can correct this at any stage, but it can often mean that you miss and electricity invoice and 'catch up' the next month.