H4 to H1B Change Of Status on Existing valid H1-B Petition

[Shrik107]

I entered USA on H4 visa, my status is H4.
I have H1B approved from previous employer valid until Aug' 2021 and want to rejoin the same employer.

1. Will this case require only filing a COS from H4 to H1B on exiting approved H1B petition? If yes , what is the timeline for processing just the COS in premium mode?

2. What is the USCIS fees for filing just the COS H4 to H1B on existing valid petition?