Hi @Meg_Middaugh1 welcome to the forum! 
Great question! While you should be able to see and use premium features ,there are certain limits to your functionality as a guest in an upgraded space. Custom fields in a project are visible, for example, however you wont be able to edit, or create custom fields. The post below might be helpful to reference in this regard.
Regard your specific question about forms, as a guest: you can see the form! In fact, the form can be shared to anyone with an internet connection. They do not even need to be a member of Asana to submit a form.
That said, premium plans are only be applied to an Asana space (as opposed to an individual account) so unfortunately you’re not able to upgrade your account for use in another space.
Let me know if you have additional questions Meg!