When trying to lower the RAM or CPU limit for a VPS under "CCP > Buy Additional Resources on step 2", the minimum allowed value to be set is always the current value. Thus, it is impossible to downgrade those two resources for a VPS.
By design POA gives whole limit of measurable resources like RAM and CPU to VPS, as a result usage becomes equal to limit. Also PBA-E does not allow downgrading resources below its usage. These two facts lead to inability to downgrade such resources for VPSes.
A feature request for adding such functionality has already been placed with internal ID POA-79465.