This isn't that tough, once you decide MySQL isn't the all powerful wizard behind the curtain. But I know I'll need this later and will forget so I'm writing it down.
The custom field name is kept field name meta_key in the wp_postmeta table. Anybody with a passing knowledge of MySQL will have all they need at this point. But for those of us that don't, here's the command you'll want to use:
update TABLE_NAME set FIELD_NAME = replace(FIELD_NAME, ‘find this string’, ‘replace found string with this string’);
So, if you've got MySQL Admin with Query Browser installed, fire up Query Browser and use:
update wp_postmeta set meta_key = replace(meta_key, 'original string', 'new string');
In my case, I had a custom field that I had labeled as Thumbnail and I wanted it to be thumbnail (petty, I know). So:
update wp_postmeta set meta_key = replace(meta_key, 'Thumbnail', 'thumbnail');
Don't forget to actually execute the query; and before you go too far, make sure that you're working with the right schema. You might want to do a select * from wp_postmeta just to make sure.
Again, this is stupid easy if you have any idea what you're doing with MySQL ... I don't so I ...
