Maaser Sheni, Chapter Two, Mishnah Seven
1) Bet Shammai says: one may not turn his selas into gold dinars.
2) But Bet Hillel permits it.
a) Rabbi Akiva said: For Rabban Gamaliel and Rabbi Joshua I turned silver into gold dinars.
Section one: According to Bet Shammai, if one has gathered together a large amount of silver coins of maaser sheni, coins which he used to redeem his produce, he should not exchange them for gold dinars in order to lighten the load that he has to bring to Jerusalem. According to the Talmud, Bet Shammai does not allow this because they are concerned that if this were allowed, people would delay bringing their silver coins to Jerusalem until they had gathered enough to turn them into gold dinars. Delaying bringing coins to Jerusalem might be problematic if this would lead to one accidentally uses them for a profane purpose.
Section two: Bet Hillel is not concerned with this problem, and neither are Rabbi Akiva, Rabban Gamaliel or Rabbi Joshua.
It is interesting to see Rabbi Akiva serving his elders. This would have been common for young scholars, who served as apprentices for their teachers.