Iphicrates almsot certainly reformed his peltasts to make them "cheap" hoplites - a mercenary peltast was paid only half the rate of a hoplite IIRC!

His defeat of the Spartans outside corinth was not achieved by these new troops, but by tradisional peltasts throwing javelins.

the later presence of "Peltasts" as elite pikemen in the Macedonian army defeated at Pydna is probably nothing more than a bit of typical military naming history - the British army still has "Fusileers", who do not use the Fusil, the American Army has "Rangers", who are not frontier scouts, etc - military names can sometimes have little or no bearing on the nature of the unit that carries them!

Possibly the later Peltasts became elite by way of mercenary peltasts beign an elite in eth army, then the guard troops adopting their name - but that's just supposition on my part.