Write a stored procedure and schedule it to run every hour that checks the latest STARTDATE in your table.  If the number of hours difference is greater than or equal...

update {your-table}
   set TPN = right('000000000' + rtrim(ltrim(TPN)), 9)
 where isnumeric(TPN) > 0

When you drop the original column, the index associated with that column will be dropped.  Then after you rename the new column to its old (original) name, the index will...

select Case_ID