H1B<-> H4 EAD <-> H1B

#1
My spouse's previous company filed for her H1B last year. Meanwhile, I got my I-140 approved. We applied for her H4-EAD based on my approved I-140. Both of her H1B and H4-EAD applications was with USCIS concurrently.
Couple of months later, both the H1B and EAD got approved. We received both the EAD card and the H1B I-797 approval notice with I-94.(Also her I-140 got approved)

She has currently got a new job opportunity which doesnt support H1B transfer. However they are ok with her EAD. How can she use the EAD which is already approved few months back? Can some clarify some of my questions below.

Is a change of status from H1B to H4 enough(visa stamping required ?)? SHould we submit a new EAD application once moved to H4. Or can we use her approved EAD card after applying the COS from H1B to H4?
Should we wait for the COS approval or just the submitting application is enough to start using EAD?
What will happen to her H1B and approved I-140
 
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