Membership Change Function

Hello,

Let me start by saying thank you for the plugin i really like it. I’m currently modifying the plugin to fit my needs. Can you tell me where i can find the function that triggers the membership update? I need to update a custom user meta field at the same time when the membership updates, if you could point me to where the function is, i should be able to trigger the update user meta function when the membership updates.

Thank you so much.

http://wordpress.org/plugins/paid-memberships-pro/