Updating MySQL Database after install

Hi,
I am wanting to get some pointers on how to update my MySQL database  
after there is already members.
For eg, I am trying to set some foreign keys that were not part of my  
database when I installed it.
There are members in that database. Can I update the database with  
the new .sql file without loosing my current info? If so, how?
Fields in the user table have not changed except foreign keys have  
been assigned to it.
TIA,

Karl DeSaulniers
Design Drumm
http://designdrumm.com