User references blow memory stack for sites with large amounts of users.

Hello,

I have a user table with 300k+ users. When going to change a CCK widget from a single select user reference to a multi select user reference I get a memory allocation error. Now outside of going into the database and manually editing the node_field table for that field and or truncating the users table (which will not be happening) how does one overcome this issue? I also am not able to increase the size of the memory allotment in php.ini as this change will be placed on a production server. This is being done on Drupal 5.7

Any help is greatly appreciated.