First Sponsorship change can only happen if you have completed TWO years with your first employer (Thereafter every ONE year). In case immigration has blocked your sponsorship change, then an alternative is to get an NOC letter from your first employer (To avoid 2 year ban), Cancel your existing RP and re-enter Qatar with your New Employer's Visa.
Pl. Note: Your New Employer SHOULD have a pre-approved Quota of your nationality to issue you a Work Visa.