Installing CA bundles is not possible on free hosting. But you don’t need to install them either: most browsers will accept an SSL certificate just fine, even if it doesn’t have the CA bundle.
You can upload CA bundles on all the premium hosting packages.
No, accounts cannot be downgraded.
The premium hosting and free hosting systems are separate, and are quite different from a technical perspective. iFastNet does have tools which can be used to migrate an account from free hosting to premium hosting, but it just copies your website from free hosting and updates the settings.
So any changes which happen on the premium hosting are not reflected back on the free account. And due to the additional features of premium hosting, there is no way to fully migrate an account back afterwards.
You are of course free to setup a free account again and migrate your website back to it, but it won’t do so automatically.