I like the thought, but I don't know if Oracle really had that in mind at the time they added the OR REPLACE clause to the CREATE TRIGGER command syntax. I believe they just wanted to implement an "update" capability. So, instead of creating a whole new set of commands like UPDATE TRIGGER they added the OR REPLACE clause to the existing CREATE TRIGGER command.
An UPDATE TRIGGER command would have been interesting ... imagine saying
UPDATE TRIGGER x LINE 47
OK, then the answer is "no." If you are modifying an ON UPDATE trigger while an update transaction is in progress, your trigger command will be queued and not implemented until after the update statement completes.
If, on the other hand, your trigger is a simple ON INSERT trigger, then while the user is doing an update the trigger can be created or replaced at any time without waiting.
p.s., I downloaded another DBForum user's SMBIN table example with all 400,801 rows to test you question. My computer was updating all rows faster than I could copy/paste trigger code into a SQL*Plus window. I therefore had to do "INSERT INTO SMBIN SELECT * FROM SMBIN" a few times to double and then redouble the number of rows. Finally, when I had 10 million rows the updates were taking long enough for me to have time to copy/paste new trigger code into the SQL*Plus for Windows. I did not have time to query the DBA_WAITERS and DBA_BLOCKERS views, which I would normally do.