Yes, you may apply for Extension of Stay on H-2A 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 renewal request.
Yes, you may apply for change of status on H-2A visa. If you are changing the job or employer (like H-2A to H-2B), 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-2A certification is issued to the employer, not the worker, 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.