With 'filling in forms' protection, you can't access header/footer content; you can with 'No changes (read only)' protection if the header/footer content control is marked as an exception. You have a number of options:
1. temporarily disable 'filling in forms' protection while updating the header/footer content;
2. switch over to 'No changes (read only)' protection with the appropriate range exceptions
3. apply a unique Style name to your PartnerName content control and, instead of using your PartnerNameHeader content control, use a StyleRef field in the header to reference that Style.

PS: with 'filling in forms' protection - even if temporarily disabled - you don't need to lock the content control, because it's inacessible.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]