You really shouldn't be using content controls and formfields in the same document - they don't work well together. Stick to one or the other.

With text formfields, you can change their accessibility by switching their properties to/from the 'Calculation' type.

With text content controls, you can change their accessibility by switching their properties to/from the 'cannot be edited' state.

Whichever approach you take will require a macro. The formfield version would be more complicated and the 'filling in forms' editing restrictions that are necessary for them to work are rather more restrictive than those needed for content controls (which may not even need any, depending on your needs).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]