As a Microsoft Dynamics user you are entitled to change your support partner at any time.
A Dynamics GP user would normally invite two or three partners to tender for the support contract. This allows you to assess personnel, skills, costs (both consultancy and support), etc.
Once you’ve selected your new partner, you simply complete a Support Transfer form, this can be done electronically or via a paper based process.
All that is left to do is for the partner to submit that form to Microsoft and you’ll have completed the change.