Make sure you have your PSP 1.5+ with you. Go to the same retail store you bought your PSP at. Also, make sure you have the reciept with you.
Tell them you want to exchange your PSP for a 1.50. Give them your PSP + the reciept for your PSP, and find the used PSP with 1.5 on it. This way there will be no money involved when exahanging your PSP. If however you have no reciept to your PSP, you might be expected to pay more $. The only way this won't work is if the stores do not sell any more used PSP's 1.50.
That's my theory, since all other downgrade attempts have been conformed fake. ;)