database - Routine to convert custom post meta from old to new value in WordPress

You can run the following sql queries:

update wp_postmeta set meta_key=replace(meta_key,'MyCustomHeader','_MyCustomHeader') where meta_key like "MyCustomHeader";
update wp_postmeta set meta_key=replace(meta_key,'MyCustomTitle','_MyCustomTitle') where meta_key like "MyCustomTitle";