I think, you can use EAD for another job. However, once you start using EAD, you will not be in H1 status anymore, even if you have not completed 6 years. What that means is, if something happens to your I-140, then EAD is not valid anymore. In that extreme scenerio, if you want to go back to H1, it is not so easy like a H1 transfer, because, it is considered as a new H1 and can only obtain only if the cap is not reached. So there are some risks in using EAD, but if you are willing to take that risk, you can always work on EAD.
I am also in the same boat, but refraining from using EAD, mainly because of the above said reasons.