Is it possible to do a volume downgrade?

When it comes to resizing your Block Storage Volumes, they cannot be sized down. You can only increase their size.

If you'd like to have a smaller Volume attached to your Linode, you can always add a new Volume of the size you desire, and transfer the data to it from the larger one.

Block Storage - Features and Pricing

More information can be found in the guide linked below:

How to resize a Block Storage Volume