If you examine the bottom of both, there is a ledge in the casting and it appears that the ring simply snaps over it. I think you’ll need to do some gentle prying to get those snaps over the ledge without breaking the part. But I’m at a loss to suggest a definitive method.
I think what I’d try first is to use a flat screwdriver to gently pry against one of the plastic lips At the same time apply gentle downward pressure to the plastic so when you release the blade from one spot it stays retracted. Then work outward from that spot to get more of it retracted. After one is free and the part is slightly canted then move onto the other plastic lips. If it doesn’t seem to be working for one lip then don’t force it. Something more creative might be required.