You can do this using dropdown formfields and formula fields in a document with forms protection applied, with no programming at all, or using dropdown content controls and a ContentControlOnExit macro. Since we don't know what kinds of dropdowns you're using, it's impossible to give specific advice, though there are examples you can find via a search on this board of Word using both approaches.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]