Save the change communication email template for a compensation planning cycle.

Only applies to a cycle whose letter is a single message. On a cycle assembled from sections the letter body lives in those section rows, so this route returns 409 with stringErrorCode CHANGE_COMM_SECTIONS_OWN_LETTER and writes nothing at all, including the subject. Check hasSections on the change_comm GET to tell the two apart before writing.

A successful save also marks the cycle's change communications reviewed, which the launch route requires before it will move a cycle to Live. Supplying the letter body counts as having read it, so a cycle created and authored entirely through this API can be launched without a visit to the web UI. Only a save that succeeds and supplies something to read does this: a 409, a rejected request, a failed write, or a save whose subject and body are both empty leaves the cycle unreviewed. Clearing a letter is still allowed and still returns 200, but it cannot stand in for having read one, so a cycle cleared this way will refuse to launch until a real letter is saved. Re-saving is harmless and keeps the original review time. The response is unchanged either way.

OAuth Scopes: planning_cycles.write