How to restrict to allow only the tier change without plan change

Need to restrict that. Other than Open Enrollment or Enrollment override life events, the system should restrict changing the plan other than a tier change. For example, a team member has a previous HDHP plan with Employee+1 tier associated with HSA coverage. During any life event change (except Open Enrollment or Enrollment override), If the plan changed from HDHP to 90/10 plan then the system should not allow it to be submitted.