Yeah you definitely don't get the upgrade back if you make a change to your booking once the upgrade has been confirmed. If you decide to change your flight, that will be done according to original fare class conditions and the voucher is lost. Same goes for if you successfully upgrade yourself via avios - you lose your avios.

Just as a data point on upgrades waitlisted for approval by revenue management, this is not possible if the only business fares left are business flex. Been told on the concierge line that they are not even able to waitlist upgrade requests when all that is available is business flex, let alone happen to get approved.