I just happened to see it again yesterday. I wish I could be helpful, but somehow everything about the key sub-plot manages to confuse me, including that part.

A locksmith can take an impression of the door lock and make a key. It happens all the time of course this was one thing - major thing that did Greenleaf in. How Columbo thought to change the lock ahead of time still baffles me though

He probably changed the lock to secure the crime scene.
This episode is overly complicated .
Strangely, Lt. seems to believe Greenleaf till near the very end!

The locksmith looked like he could have been Columbo's brother. Same height. Same hair. Similar face. Same overall impression.