If your Qatar Id. is issued then you have to work in current company till next six months from date of issue of your id.
After six months, you have to get إعارة (Job Secondment) letter, by this, you will be still on your current company visa but can legally work in another company.
After twelve months completion with current company visa, you join new company on new visa through لا مانع / شهادة عدم ممانعة (NOC) or transfer your visa to new company through نقل کفالا (transfer letter) & NOC
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