if you convert JPY you've earned in JPY over your lifetime, to USD, I don't think you calculate some gain/loss based on hypothetical cost basis of that JPY in USD going back forever.
if you move assets or currency back and forth from USD then you might generate FX gain/loss but see Section 988 and exceptions for personal transactions.
It's a tricky area, you might want to get another CPA opinion for your situation.
