First of all make sure to read the Microsoft Dynamics Ax whitepaper for writing upgrade scripts, this will give you insight of what kind of attributes you need to set on  your upgrade method (Pre-script, Post-script, tables you perform CRUD operations and so on). After you have created the upgrade script, you are good to go, once you import your new model you just need to run the Software update checklist (this is needed in order to register your new script) and then after that the Data update checklist, which performs the update.

If you have questions on this, let me know: fabnor@dynaxti.com