If it is just a change of sponsorship, no exit is necessary neither your ID number changes.
Things do not remain this simple once the 'losing' employer cancels your visa and offers an NOC. You will have to take an exit (it could be a hop to and back from Bahrain) within 7 days when this happens. However the 2 years ban does not apply on you since you have an NOC from this employer and a visa from the 'gaining' employer. Because you now have a brand new visa, your ID number can not remain the same and your profession might also be different from now. You'll have to take all those procedures (medical, finger print etc.) that you did when you entered Qatar the first time.
If it is just a change of sponsorship, no exit is necessary neither your ID number changes.
Things do not remain this simple once the 'losing' employer cancels your visa and offers an NOC. You will have to take an exit (it could be a hop to and back from Bahrain) within 7 days when this happens. However the 2 years ban does not apply on you since you have an NOC from this employer and a visa from the 'gaining' employer. Because you now have a brand new visa, your ID number can not remain the same and your profession might also be different from now. You'll have to take all those procedures (medical, finger print etc.) that you did when you entered Qatar the first time.