Hi Channa,
You tell that you got a work visa and are yet to travel to Qatar which means that you just have a paper visa with which you can only travel into Qatar. For work visa to get stamped, it is a long process. After you arrive, you need to do the blood test and X - Ray. After you get this report, you go for finger printing. After that you will get a visa on your passport and get an Qatari ID Card. This process takes anything between 20 days to over a month. So if you have not travelled yet, you can straightaway go for any offer you get. But if your ID has come that your sponcer's NOC is required for any change.

Cheers !!!