Here's your options... Use one or more of these tips to solve the problem.
1. Stick to the same size iPhone when you upgrade, and updgrade every year or two. Unless your other half loves iPhones as much as you, they're probably not going to notice you've had a new phone... And if they love iPhones as much as you, you should both upgrade and then that solves the problem...
2. Get the same case as your old case for your old iPhone. Further stealthiness...
3. Say work got you a new phone...
4. Buy them a new iPhone, too... As their main Christmas present. Two birds, one stone, and all that. Get a monthly (ideally interest free) payment scheme of some kind if you need to spread the cost. Multiple iPhones gets expensive.
Good luck!