My petition is approved in 2016 but not been used till date. Will it be valid or what need to be done for H1B stamping.
Your H1B sponsoring employer will need to file a new cap-exempt petition and once approved you can use that for visa stamping and travel to the US to work for the sponsoring employer.
Still possible, if I have changed the company.
Yes as your H1B was approved previously, you new employer can apply in cap-exempt.