It is not possible to switch a subscription's overuse policy from "Overuse is allowed" to "Overuse of disk space and traffic is allowed" if the traffic or disk space on the subscription already exceeds its limit:
Error: Unable to change resource limits. Unable to set the required resource usage limit (100 MB disk space) because the new value is less than the actual amount of resource already used by your account and sites (102 MB disk space).
The problem seems to be caused by an error in the product and is currently under investigation by the Plesk Service Team.
Right now, it is possible to switch the overuse policy from "Overuse is allowed" to "Overuse of disk space and traffic is allowed" only if the offending limit (disk space and/or traffic) is increased to be above the current usage.