Back in the day, late 60s early 70s, I think the conversion was accomplished by taking the "disk" (or the equivalent) that had an orifice in it that was inline with the gas supply and flipping it around. Flow in one direction was for LP, the reverse was natural gas. Memory is hazy, but I'm fairly sure I did this conversion for my dryer then.
Do I remember correctly?
I only have conspiricy theorys to explain why this is no longer the case.
charles