Unfortunately you'll have to replace the headlight housings themselves if you want to get rid of the now-yellow rings. The damage is permanent, and also well documented across many series of BMW's running HID AE's (Starting with LCI E60's IIRC).
The good news is that with so many people preparing to retrofit LCI F01 headlights the price of used housings will become exponentially cheaper in the next year or so. If I were you I would just keep on doing what you're doing and when you replace the headlights eventually just keep the incandescents (maybe run PIAA) or go LED. Aside from actually installing LCI headlights with the advanced LED lit halo's, you'll be hard pressed to match the look without consequence and/or sacrifice.