if or case statement with nested select statement

I have three columns member_ID, Effective_Date and Termination_Date. Member_ID is not unique.

I would like to do the following:

Select
   If Termination_Date is null 
      then select min(Effective_date)
          where current_record.member_ID = enrollment.member_ID
          having min(enrollment.effective_date) <> current_record.effective_date 
      Else 2014-12-31 
      end as updated termination date
   from enrollment 
   where Current_Record.Member_ID = Enrollment.Member_ID        
from enrollment