I have a question of Pro site plug in

I have installed pro site and WHMCS plug in , and that two plug-ins works great .But I have some question about it :

1) My goal is offer users to pay and get the different level which is silver and gold at my pro site. But if i have a plug in( Formidable pro ) wanna using within this two levels but the contents is different . For example in the Silver level it has a Formidable Form content page 1,2,3,4,5, and the Gold level it has the content 2,3,6,7….

so how to achieved it ?

I figured it out this if I using membership plug in.But how to do that ?

In future when the users upgrading or downgrading the level how unaffected ?

2) How to my user wanna to upgrading or downgrading the level from my pro site ,is it wanna order at their WHMCS account ? Because WHMCS has handle my registration and payment system.