Can I legally put the later model pants on it? If so, what's required in
terms of paperwork? 337? Log book entry? $1500 in unmarked bills to the
local FSDO?

Not telling the local FSDO ?

Seriously, I doubt this needs any kind of approval, if the parts are
Piper manufactured (or OEM) and the model years in question share the
same type certificate, I wouldn't worry about it too much...