Porting old number after tryout?

[newz]

I have been using the old nokia 5185 on at@t's prepaid network. The phone and service has been very good but I really need a smaller phone. The size of this phone is embarassing. Att doesn't offer any decent phones on prepaid so I would like to try virgin mobile. They have a highly rated audiovox 8610 and their service uses sprint which is ok in Los Angeles. Question....I would like to buy the phone and try out the service before porting my old number. If the service is poor I don't want to port the number, then cancel virgin mobile and lose my old phone number. Can I port my old number after I activate with virgin ...a few weeks after I try the service? I think virgin gives you 30 days to check out the phone.