MS Project - Immutable Task Identifier when rescheduled the programme

I'm working with MS Project 2010, importing project files to my web application using MPXJ library. When my clients reschedule it, I need to identify which tasks was already imported, because in the web application, additional data is added related to the task. So, I need to keep the historical data integrity.

I try with ID and GUI fields, but none of them is immutable when the programme is rescheduled.

Which task field don't change when the programme is rescheduled?

Thanks in advance!