You need the service wrench. P/N J10014905 / this has two slotted sides that will fit in the openings in the brass retainer rings. One is smaller and one is larger. I've heard of but never used a Dacor made wrench that is similar and a little more ergonomic.

They used two separate types of metal, brass rings and aluminum venturi. They tend to seize...I mean, they are always seized. Proper tool and force is the only way. It's quite possible that a venturi or two will be broken, this is no ones fault but be prepared to possibly need to replace them. *edit - when re-installing everything add some anti-seize paste on the threads. it can be found at a hardware or automotive store. It's in a grey/silver squeeze tube about 2-4oz or larger jar. This will prevent future seizing, but not be of much help for what's already there*

Hope this helps and sorry i didn't see in time to get you this before Christmas.