B1/B2 to H1B transfer possible while in USA?

I am currently in the USA on a B1/B2 visa and have received a job offer from a US Employer.

  1. Can I get H-1B Visa for next year right away in Dec?
  2. Or I have to file an H-1B visa in April because I have read next year’s H-1B cap has already been filled for those applications in previous April.

Job interview for H1B while entered USA on B1/B2 Visa

Since you entered US on B1/B2, I am assuming that you came here for tourism.
Legally, you cannot give job interviews on B1/B2 visa.
Your job offer might be viewed as suspicious under these circumstances.

Cap subject H1B

The H1B cap opens every year on April 1. You cannot get your H1b in current year quota if the lottery has already been closed.

Employer files H1B in new season in April every year

Your employer will have to file for your H1B in April for new fiscal year quota.

Stay in US with B1/B2 visa - File H1B and wait for approval?

I strongly advise you to leave USA before filing for your H1B to improve your chances and reduce the visa intention conflict.