What will happen to my content if I downgrade my current premium plan to free?

Downgrading restricts you from using certain features that are not part of the plan you opt for. If a Pro user downgrades to free membership, he will not be able to enjoy features that are a part of Pro membership. Also, his private presentations will be blocked or deleted. Similarly, if a Business user downgrades to Pro, he will not be able to enjoy the features that are a part of the business plan. For instance, his branded channels will work but without branding. For more details, see here.