One of my friend came to Qatar on july 2012 in work visa. Currently he got offer from other company (oil & gas) & His current sponsor willing to give him NOC. As far I know Qatar labor law allows first sponsorship change only after 2 yrs .But some overhearing that this 2 yrs time will not only be applicable for company to company sponsorship change .
If have NOC from the Company you can change sponsorship . no problem
One of my friend change before 2 years
as far as i know that after two years you can change your sponsor but if they agreed to give you NOC otherwise they will cancel the VISA and no transfer will be made , it is all based on the agreement between you and your employer