I am on F-1 OPT status and will apply for H1B this coming April 1st. When is the earliest date I can change employers and not have my H1B application revoked?
Your H1b application has nothing to do with your OPT employer. You can change employers any time as long as they comply with the OPT criteria.
If you mean that the employer you work for now is the one applying for H1b, and you want to leave that employer - then you should probably wait until the application is in fact approved, and then the new employer will file a new H1b petition based on the quota already allocated to you.