Could confirm this.

Few days ago I was clarifying usage of miles and/or vouchers for upgrades and was calling LH Senator line.
It was specifically highlighted to me by agent, that:
- when ticket is upgraded "in advance" either using miles or vouchers online or via phone, the booking class would be updated accordingly. This would bring as well updated collection of miles as of new updated booking class;
- when ticket is being upgraded at the counter, either checkin or at the gate (miles, vouchers, paid) or even in the plane by crew (most common way - paid), the booking class and mileage collection remains as of original booked class.

Hope this make it more clear.
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