Do I have to go back and re-account for Initial Direct Costs on leases that existed prior to my transition to ASC 842/IFRS 16?

Not necessarily. The FASB and IASB have provided organizations an expedient allowing them to not have to go back and re-account for these costs. This practical expedient is referred to as the "Package of Three" which is further explained here. 

This is only required for leases entered into on or after the organization's transition date.