I don't know "legalities", but if you are a local, wouldn't it be easy to make an amendment to your Constitution? After all, the U.S. Constitution has a few!
1989 isn't that old so it would be a great time to adopt something that may strenghten your bonds "with the outside" world.
If you are thinking about going national someday, the alumnae initiation eligibility might be one of the factors that may influence your decision on one group or another.
Best of luck!!