IANAL, but this is a generalization and not true in all cases. I think they might be able to try to force you not to use your own code if, and only if, the CLA requires you to assign copyright to the project. I think it would still be on shaky ground, as you could reimplement what you did and be fine. You as copyright holder have the right to re-release code you solely hold the copyright to under any license. I will sign a CLA that requires me to affirm I have the right to contribute the code, but I will not sign one that requires me to assign copyright, unless I am getting paid (in which case it becomes a work for hire).