No. While upgrades to larger plans are possible at any time, downgrading to a smaller plan is not supported.
Larger plans include larger disks, and virtual disks cannot be safely reduced in size. Unlike extending a disk, which simply adds new blocks, removing blocks would truncate existing data regions. Since there is no reliable way to guarantee that no filesystem metadata or user data resides in the area to be removed, shrinking a disk would almost certainly lead to filesystem corruption and data loss.