What is your budget? What amp are you using? How loud do you play? Have you reached the limits of your Zoot cab? Do you use a DI or mic your cab? Are you expanding because you want to or are you playing louder and need to?
If a standard 115 (assuming 8 ohm) has been good for so long, just adding a 210 will be the simplest and cheapest solution.
With 1 good 212 you shouldn't need to keep the 115 at all. The 115 won't add much in that set up because a good 212 can do everything you need on its own. Upgrading to 2 212's will be like chopping in your Ford Focus for a Ferrari.
You could get another, better, 115 and replace like for like but if space is a limiting issue then I'd be looking at 1 or 2 of the smaller, more powerful 112's.