Does checking in online the day before your LH flight make it easier or harder to apply an upgrade certificate when you arrive at the airport? The certificate says "see the Lufthansa agent at check-in to request an upgrade"; a strict reading suggests that you need to check in at the airport and present your certificate when you do that.

I'd kind of like to check in and pick my Y seat in case the upgrade doesn't clear, but will that make it more of a hassle to apply the certificate when I get to the airport?