You CAN change sponsor after 1 yr(not 2 yrs)- of service with the same employer/sponsor as per Immigration Laws even if you dont finished your 2 yrs. contract.

Provided that, your sponsor will give you NOC/Release. BUT, even if you worked with your sponsor for 10yrs or more, if he refused to give you release to another sponsor and cancelled your visa from the Immigration., you will be banned for 2 yrs. to enter Qatar.

On the other hand, even if your sponsor will give you release within 1-yr. of working under his sponsorship. You CANNOT change your sponsorship to another one.

That's Immigration Law!;)