H-1B Transfer approved and yet to switch employer due to COVID and current H1 needs Extension

[uchase4me]

I am Currently working for employer A, applied for H1 transfer to employer B and got approved in March,2020 for 3 years.  
But I didn't switch the employer due to COVID situation.
Present H1 with employer A is expiring soon and needs to file extension.
If I file extension and get approval, can I switch to employer B in future with the prior application?

Can some one please advice if it is legal and the approved transfer H1 application is still valid?

[gopalakrishnach]

I feel there is time limit you need to join employer B. if not employer B must revoke H1B or else pay you.