H1 extension has filed with I-140 approved

[jain.atishay]

Hello,

My I-140 is approved 7 months back. And my H1 extension has been filed couple of months back. couple of weeks back, my stamped VISA also over.
If my employer cancels my H1 extension filed, what can happen?
With new I-140 rule, I have 60 days of grace period.
In this grace period, if I find another employer, will they file H1 transfer or new H1?
How long I have to wait to start working with new employer?

Thanks