I haven't had to do this yet, but from the looks of the retainers:
I would put the grill, with chrome moldings face down, on a blanket or carpet and use a long screwdriver (or other suitably long skinny implement of destruction) to bend the plastic snap tabs over, intentionally damaging them. Sure, you'll have to get new tabs, but better to spend $20 for tabs than $100+ for molding.
There may be a less destructive way, but I can't think what that would be.