You cannot switch back to Momentum, as they only support their current plans Points Plus.
If you aren't losing, you might have to look into what you're eating VERY thoroughly. WW has always allowed people to eat what they'd like, but the new plan puts a much larger focus on filling/whole foods. Not to mention some people are confused by Zero Points fruits. Fruits and most veggies are zero points, they are not "free for all". You still have to keep them moderated.
I'd suggest you try at least one more week with Points Plus to see what's going on.
~Made of star stuff~