Dfyjkt

I'm planning a trip to Iran, and most (if not all) the money I can bring would be in Japanese Yen.

Will it be easy to exchange ?

Every guide I read says that EUR GBP and USD are widely accepted but this not so helpful for me.

You can exchange Japanese Yen in Iran, but it may be at less favorable rates and fewer locations.

Farsinet explains:

Iran has two exchange rates, one formally fixed by the Central Bank of Iran and another informal, open market rate that's sold to the public via currency exchange shops and traders on street corners (Street Rate).

Iranian Rial rate changes frequently but, on January 26, 2017, here's where it stood, per Farsinet's data:

Official => USD 32,360 =>Street => 38,490
Official => Euro 34,806 =>Street => 42,100
Official => British Pound 40,965 =>Street => 48,900
Official => Japanese Yen 285 =>Street => 345
Official => Swiss Franc 32,406=> Street => 39,000
Official => China Yuan 4,704 =>Street => 5,600

What this means is that it may be more difficult to find an exchange shop or trader that will accept yen and, when it does, give you the best rate, as they give preference to other currency.

Again, Farisnet explains:

The Unofficial-Street Exchange Rate used among small businesses and private transactions was much higher than official multi-exchange-rate prior to 2002. Since going to a single exchange rate the official and "Street-rate" have come much closer. The rates listed are based on the official exchange rate posted by the Iran Central Bank. We compare the official and "Street-rates" to ensure rates listed are the most accurate indicative of the true IRR-USD exchange rate. US Dollar (USD) is the most widely used foreign currency and the new $100 bills are preferred for street transactions and get the best rate.