When a product has active subscribers, you will be prevented from making changes to the product price, shipping date & product variants.
This is prevented in order to ensure that subscribers are not charged in excess of their current agreement, unless otherwise communicated by the seller.
That being said, if you want to make changes to a product with active subscribers, we advise you to first create a new product that has the proper settings.
You'll want to make sure that your shipping date, billing date, variants, and all other areas are configured to your liking.
After confirming the new product is set-up properly, you will need to move those active subscribers over to the new product.
In order to move active subscribers from one product to another, you can use this guide to help you do so: Move Subscribers to a New Product