sim card from iphone to blackberry bold 9700?

I have the 30 dollar iphone plan. if i buy a bb bold 9700, could i switch the sims and put it into the blackberry and then like switch the plan online w/ mywireless account? (if you can do that) and then if i want to use my iphone again, just put it back into my iphone and switch back (if you can do that too) lol i’m just confused and i’m getting a bunch of differeht answers. please help me. So like will it affect my current plan? OR do i have to like switch or something every time I would change the sim between the two? HeLP