Bulk change of group membership

I have about 500 users on an NT4 network.
Every user has a login name that starts with a number and then has 2 letters follwed by 2 numbers. e.g. 88ma11
All users starting with the same 2 numbers for example the 88 are members of a a group called year7. All users starting with 87 are members of a group called year 8 etc etc.

At the end of the academic year all the users starting with 88 need to move to group year 8 and all users starting with 87 need to move to group year 9 etc. What is the easiest way to move groups on mass please.

Thank you for any help.
Please email to mikealger@hotmail.com

Mike