mogman, concerning changing an engine from the pop

mogman, concerning changing an engine from the poppet valve system to a CSRV valve system, would you please attempt to explain the difference between a "conversion" and a "retro fit"?

I am thinking that a conversion involves parts that have already been made to fit the engine and a retro fit involves having to make the CSRV valve parts from scratch to fit a different engine. Is that anywhere close?

Thanks. Holding on to what I've got!