L1B to H1B COS Different Employer

[ravikrishnaabcd123]

I am currently in US on L1B from Company ABC valid till DEC 2020. And I have an approved H1B from Company XYZ which was never used or stamped. This H1B was applied in 2015. I want to know:

1. Can I apply for H1B COS to Company XYZ?
2. If appled and denied, do I have to leave USA or can I stay on my current visa?
3. And If approved, will I get any grace period to serve two weeks notice with my current employer Company ABC before I join Company XYZ?

Please let me know. Thank you.