Thanks, Zorn.
More complicated that I thought
I note sometimes when I fly YYZ to FRA on a LH operated flight, the ticket uses the LH flight number.
Other times, the ticket uses the AC code share flight number.
In both cases, the electronic ticket number starts with 14 which I understands means an AC ticket.
I wonder if this affects when one can change from LH "metal" to AC "metal"