how to overwrite a propety by a plugin?

I read the "plugin tutorial" and can write a plugin to overwrite a simple module, but when i want to change some thing in the app/models/issue.rb:
Remove: belongs_to :assigned_to, :class_name => 'User', :foreign_key => 'assigned_to_id'
change by: belongs_to :assigned_to, :class_name => 'Principal', :foreign_key => 'assigned_to_id'

I don't know how to do that, because it not in a function ?