There are a number of limitations on how these Miscellaneous Charge Orders (MCOs), or "travel credits" as you put it, can be used. They are set up to be used as either part or full payment towards a BA published fare and must be spent via a BA sales channel.

What I know is, MCOs cannot be used in conjunction with redemption type tickets, say for example to cover the cost of taxes/fees/surcharges associated with a reward ticket or to cover the uplift in fuel surcharges when upgrading using Avios.

I suspect the answer to your question is dependent on two factors:
- Who issued the ticket - BA sales staff are usually reluctant to touch bookings issued by sales channels other than its own.
- Do airport upgrades book into published fares?

Sorry, it appears I've answered your question with further questions