I've got a flattop that I converted to a NG hookup but now I am giving to my father in law who wants to use it with propane. Do I need to replace the orifice back to original?

I'm thinking that it's more important to increase the orfice for NG since the particulars are larger in size and by switching back to propane is not a big deal.

Yes you need to replace with the original

The orifice is smaller for propane because there is more BTUs per volume than NG

OP, if it worked like that they would just sell it with the NG orifice and it would work for both propane and NG. That's not how it works though.