AC is wrong. An LX flight number would not make this an LX ticket. I assume this is not the only flight on the ticket? If there is at least 1 AC flight you can still get an AC ticket easily and technically for AC not even that is necessary.

be prepared to pay change fees and possibly higher fares - this is a voluntary change and not ‘free’. UA offers this ‘free’, but it is an exception due to their close relationship with the LHG.

LX can’t do anything here, only AC can change that flight number.

You can still buy an upgrade at (OL)CI with a codeshare, but buying up prior to check-in needs a native flight number. Also, no bidding on non-LHG tickets, so that won’t rework irrespective of the flight number. The LX number will allow purchasing an upgrade.