changing sponsorship is always difficult, price increases each time and you are increasingly regarded as a possible problem. Employers here dont like giving NOC, feel it is giving away something to their competitors! After two years out of Qatar it is possible to come back without NOC, but also consider that they may not want you! If a single woman you are a problem, if married you might want to bring ur family etc. etc.
You will be fortunate if ur spnsor will release u but this is not usually the case. Tread carefully!