On ASE 16 I verified that "create or replace" works as expected for the sa login who created AND modified the trigger.
This does seem to be atomic operation.
Worst case scenario you may have to drop trigger(s) once.
And thereafter it should work.
I will try to test/verify other scenario where we have two different logins aliased to dbo.
One creates and other modifies an existing trigger.
In ASE 16 sa/sso should be able transfer/take the ownership of an object as needed.
For triggers base table ownership needs to be transferred/taken.
Command for that is "alter.... modify owner"
HTH
Avinash