The only time I've known an FB Silver to get access to the lounges at CDG is if they have been travelling on a full fare flexi ticket (in the instances I know of, that was travelling Y). In those cases the lounge access was allowed for the outbound leg only of a return departing from CDG, and not the return. A few of my fellow travellers have done this; not sure whether or not as an Ivory this also works, or even if it's still possible (I know it was until August this year) perhaps others in the forum have similar experiences they can share...

Not really applicable for you I guess because in your case travelling Y you'd already be in TC and not be looking to upgrade, but maybe a useful bit of info for other peeps.

Cheers

Sim