Just guessing here, but if you heat the cast iron with a torch it should expand enough to get it off.
Edit: Be sure to heat it evenly or it may crack. You might try firing the egg up as aawa says and crank it up to 500 degrees or so first to see if that works.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle