When you downgrade your premium plan, the videos that have been exported/published previously will remain the same. You can access the video projects and make changes anytime.
However, the premium assets available with your previous plan will no longer be accessible.
For instance, if you downgrade your plan from Pro Plan to Free Plan, you will still be able to access all the videos you have created, but the premium assets and benefits available in the Pro plan cannot be applied to the Free plan.