I have converted to fulltime before. All contracts have clauses regarding this type of things because it happened a lot. Check your contract. If your agent is a good one, they must have such clause. If they have none such clause, you have "right to work", means you can work for whoever you like. In my case, my clause said something like company C will have to pay B some money, a percentage of your salary as a finder's fee, etc. Hope this helps.