Find non UID and nulls, calculate based on last highest UID in data and write to existing gdb.

I've looked all afternoon for the answer to this probably simple task. I am working with a feature class in a gdb. The feature class has a unique_id field that may may be populated with duplicate numbers or be null. I need to find all the dup ids or nulls, calculate them incrementally based on the last highest unique id while leaving the other records alone. I was able to put something together that seems to work when if I set the writer to overwrite existing gdb. Is there a way to do all of this and just update the existing gdb? I don't want to loose the other feature classes and tables in the gdb. I'm pretty new to FME!