I don't think you'll put lots of weight back on- only if you eat lots of processed vegan foods. If you're sensible you will probably find little change in your weight, but you will feel great. I did a 30 day vegan trial in May and stayed vegan. I still only miss butter now and then.
I would look at Defiance's website (think it is called be-defiant.com). She is a vegan athlete and probably has relevant information on there. I think she promotes the Sun Warrior protein stuff.
Good luck!