For most situations, give at least a 10-day notice before decreasing benefits. See 0026.12.03 (10 Day Notice). There are some EXCEPTIONS. See:

If clients report a change which does not result in a change in benefits, send them written notice of no change in benefits.

Notices of change in benefits must contain the information listed in 0026.03 (Content of Notices).

Adequate notice of an increase in benefits is enough. Clients can get increased benefits even without adequate notice.