That is really strange. It is still your TA’s ticket. They are using commercial inventory for the upgrade. They absolutely can change the ticket but they will need to ask BA to reinstate the voucher. The only upgrade situation where the TA would lose the ability to service the ticket and BA would take over is when Avios are used for the upgrade. I can’t think of a reason why your corporate TA would not touch their own ticket. I can understand why BA would not touch it though.