Okay, I believe I solved it.
The stupid apple autocapitalize caused the i to turn into capital I when I wrote my mysql query, and thus the serialized string that had been replaced in the DB was actually:
a:1:{I:0;s:8:”Charcoal”;}
Unbelievable, it’s actually insane that sequelpro allows autocapitalization/autosuggestion in a query anywhere in its interface.
So in case anyone does need to do something like this, I believe the following process does indeed work:
1. Alter the choice as you normally would by editing the custom field
2. Perform the mysql query with the length of the strings for the serialization in mind in the s:X: part of the query
UPDATE wp_postmeta
SET
meta_value = REPLACE(meta_value,
's:5:"Black"',
's:8:"Charcoal"')
WHERE
meta_key = 'colour'
