Good and smart question. You've done your homework. Yes, you can use your existing EAD BUT if at all possible, also port (transfer) your H-1B to a new employer so that in case something goes wrong with your I-485 at least you remain in status on your H=1B and you have more options then letting your H-1B lapse and no longer have a status to fall back on. I would apply for a porting application of the H-1B as well. Good luck.
I agree with my colleague here. Porting your H-1B via AC21 is the preferable option here. Let me know if you would like our office to assist you.
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