Yes, you may apply for Extension of Stay on H-2B visa. The employer may apply for re-certification for an additional two years with one year extensions, but on each new application, the employer must justify the reason for the H-2B extension request. You may also extend by changing to a new employer who had completed the temporary labor certification.
Yes, you may apply for change of status on H-2B visa. If you are changing the job or employer (like H-2B to H-2A), have your new employer file Form I-129, Petition for Nonimmigrant Worker at the USCIS Regional Service Center with jurisdiction over your residence.
Note: H-2B certification is issued to the employer, not the worker, is not transferable from one employer to another or from one worker to another. Certification is issued only for a specific job opportunity, for a specific number of workers, and for a specific employment period.