The primary impact of this change is to compiled add-ins. Office add-ins (add-ins using the JavaScript-based APIs) are not impacted by this change.
The work required to update your add-ins for 64-bit will vary based on the code.
For example, any use of VBA’s DECLARE statement to load external functions in compiled dynamic libraries will require that those libraries support 64-bit. The VBA code itself will continue to work as VBA is transitioned to 64-bit, but you’ll need to update the external libraries. Source