Can't modify $post['signature'] in plugin...

[BirdOPrey5]

I can modify $post['message'] and $post['title'] no problem... I'm trying to modify $post['signature'] by searching for a string, if it's found, replacing it with a different string.

I tried postbit_display_complete first and while it worked for message and title it didn't do anything for signature...

So then I changed it to postbit_display_start and nothing from there either- I get no errors but the text never gets replaced.

This is frustrating because I have another custom plugin running on the same hook which modifies $post['signature'] already and it works. (I disabled that plugin while testing the new one.)

The working plugin evaluates a template on the signature start template hook and then "erases" the existing sig with the line:

$post['signature'] = '';

and like I said that definitely works. I can't figure out what is different in the old plugin that it works where as the new one doesn't. Is there something screwy about $post['signature'] I'm missing?