Can we downgrade oracle database form 12cR2 to 12cR1
You can downgrade both major releases and patchset releases, based on the original Oracle Database release from which the database was upgraded. Major release downgrades are supported back to Oracle Database 11g Release 2 (11.2.0.2) and subsequent 11.2 releases, and Oracle Database 11g Release 1 (11.1.0.7). Patchset downgrades are supported back to Oracle Database release 11.2 patchsets, except release 11.2.0.1.

This link provides a summary of releases supported for downgrading.

Also, check this link for compatibility.