I would ask if she would like the camera as a gift instead of you trading it in on the new one. You can always add a little something to have a surprise on the birthday.
There are women that would be offended by a used gift, but IMHO those would be the ones to avoid for a LTR.
You need someone that likes your old stuff and is smart enough to look at the value of the gift - not if it is used or not.
George