The InnoDB table options page_compressed and page_compression_level were introduced in MariaDB 10.1.0.
An ALTER TABLE operation that changes these attributes unnecessarily causes a full table rebuild. Enabling page_compressed could be done instantaneously (not resulting in immediate space savings). The FORCE keyword could still be used to cause the table to be rebuilt.
Disabling page_compressed must continue to require the table to be rebuilt.
The page_compression_level basically matters for compression only, not for the decompression. It should be fine to change the compression level at any time, without any change.