Power to Choose is a program run by the Public Utility Commission of Texas. Its goal is to protect residents of the state from unfair energy costs and unregulated REPs, as well as develop a strong infrastructure. The program provides an easy to use, online tool that give residents of the state the opportunity to compare rates, plans and other energy options. Keep in mind though, you really need to read the fine print if you decide to use Power to Choose (or any other service, for that matter).
You can switch power supply providers to find more agreeable terms, snag a better rate or get green electricity. If you have a month-to-month plan for your power supply, switch at the end of any billing month. If you are locked into a longer contract and want to exit early, you may want to investigate what fees may be assessed if you switch. Power supply will be the same when you switch power providers, so you don't have to worry about sub-standard electricity supply.
In Houston, 0% of people have switched to a plan that has some renewable energy component to it. Another 0% have switched to a plan that is partially renewable, while 0% have switched to a plan that powers homes completely by renewable electricity. This of course means that 100% of people have remained on a plan powered by traditional sources of electricity such as coal or nuclear power.