Spaces and Volumes are two different products.
Volumes are Block Storage product, allowing you to attach additional SSD to the Droplet.
Spaces are Object Storage product. Spaces aren't associated with any Droplet, you can access anywhere, using any S3-compatible utility.

No, there is no out-of-box way to switch from Volumes to Spaces or vice-versa. However, there is many tools available, that can help you to copy all data from one to another.
s3cmd is the most popular utility for interacting with S3-compatible Object Storage (Spaces).
s3fs-fuse could be useful if you want to attach Spaces as a disk.

You can take a look at DigitalOcean's Object Storage-related tutorials to learn more about Spaces.