The gemara is not conclusive whether there is a gezeira d'rabonon on items that were exchanged for issur hana'ah (or whether it is only assur by a neder when he uses a language that implies inclusion of even the exchanged items). The Ran writes that because the nature of the issur even if it exists is only d'rabonon and the gemara leaves with a safek whether the issur even exists, we can apply the rule of safek d'rabonon l'kula. Both the Poras Yoseif and the Mitzpeh Eisan ask that a neder is a davar sheyesh lo matirin, and by a davar sheyesh lo matirin we are not meikil out of safeik, so why should we say safeik d'rabonon l'kula here?
1. Mitzpeh Eisan - We are only machmir by a safek d'rabonon sheyesh lo matirin when the question is about what actually happened (sfeika d'uvda), but if it is a safeik in the halacha (sfeika d'dina) then we can rely on safeik d'rabonon l'kula. Perhaps the rationale is that normally we are machmir because the fact that it will become mutar later and the entire question of what happened will not be relevant, we must wait until late. But here the halachic question as to whether there is an issur on chilufin would still exist and be relevant for other cases, even if in this particular case the neder is removed. For example, we don't know if the egg is a yom tov egg and by waiting until after yom tov the halachic question will be avoided because from a halachic perspective it no longer makes any difference when the egg was formed. But when we have a safeik in the halacha, even though the safeik is yesh lo matirin through some sort of pe'sach or charata, the halachic question will still apply and is still pertinent for the next time. Since the question will never disappear, we can rely on safek d'rabonon l'kula.
2. Poras Yosef - He gives a much simpler answer. Since the quetion is not limited to nedarim, but is rather a issur hana'ah question. Even though by nedarim this question will manifest itself as a davar sheysh lo matirin, but once we are lenient by other issurei hana'ah to pasken that the exchange is mutar, we are lenient by nedarim as well.