Expiration Confusion

[Allan McGuire]

When creating my key, it was originally for testing purposes and I set
an expiration date.  Well, it became permanent and now I want to change
the expiration date to "never".

I have been able to set the "pub" key and the "sec" key and "ssb" key to
"never" but not the "sub" key.  The manual seems to say "The expiration
time is updated by deleting the old self-signature and adding a new
self-signature".  How do I delete the old signature?  Do I need to 
"expire" the "sub" key?  If so, how do I select it to do that?

 From the manual and the bits and pieces that I got from the discussion 
threads, I am a little confused on how to do this and what commands to 
use.  I have already destroyed my key once, but I am very thankful for 
backups.

Thanks
Allan