I’ve done it several times at the lounge entrance. Last instance was last month at The Wing J. One time they asked if it was for retro-claim (RIP 2027) due to upcoming membership end date (RIP 2027) and I fessed up to it, but they didn’t kiss up a fuss and removed it. No problem with priority checking, lounge access and priority boarding (since you already have the lounge invitation and the FFP printed on the physical BP during the pre-boarding check). Worst case is you can remove the FFP after you use the lounge on your way to the gate. No PR on the flight however.

Caveat is that I’ve only done it in HKG with Cantonese staffers.

Also, to make sure your FFP doesn’t “stick” to your boarding pass, you can check the CX app for the current flight segment. Once the FFP is successfully removed, your name should not have the membership tier colour framing your name. Potentially, your itinerary will be marked as “Companion” (I think it’s basically the system saying that your account has someone else’s itinerary added to it, and they just happen to have the exact same name).