First of all, greencard is employment based. so you need a permenant job to be elligible.
Secondly, I485 can go through without any RFE. In that case, you would be ok. But if INS issues RFE, chances are you need to provide current company information to support your AC21.
If you have to get the job. According to the current process speed, INS wouldn't touch your I485 in 9 months. By the time they issues, if any, RFE, you would have a permenant job. So it is not so risky afterall.
The best way is to persuade your contractor to make it a permenant job as soon as possible. Since they give you W2, there should not much a difference to them money wise. They need to provide you a letter stating the position is permenant.
If you change your job, you do not have to tell INS that you change job on AC21. They will request your new company information. Better wait for the RFE if they issue, then tell them you use AC21.