The ‘How to book the return leg with 2-4-1 companion voucher’ thread

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Hi all – I am looking to fly at LHR – JFK return for one person. There is business availability outbound but only economy inbound. It seems like more seats become available in other classes one or two days before.

If I was to book business out and economy return using a companion voucher, would I be able to upgrade the inbound journey if I have already flown the outbound leg? Not sure what the rules are of using the companion voucher in this way.

Thanks!