The answer depends on which fruit you ate. Before we explain this answer, we will begin by summarizing the Levush’s[1] overview of the various berachot acharonot and of what to do if one recited the incorrect one.
Birkat HaMazon is a Torah-level obligation,[2] prescribed by the Torah for bread, which is both filling and the staple of a classic diet. The Rabbis established a Birkat HaMazon-style beracha (Me’ein Shalosh) for the seven foods for which Eretz Yisrael is praised, which are mentioned in the p’sukim[3] around the pasuk about Birkat HaMazon. (There are opinions that the Me’ein Shalosh beracha is also a Torah-level obligation.[4]) Within the versions of Me’ein Shalosh, the highest level (and thus the first mentioned when one makes a beracha on multiple Me’ein Shalosh foods) is Al HaMichya, because it is recited after eating grain-based foods, which are generally more filling than fruits. Al HaGefen for wine is next in importance, followed by Al HaEtz for grapes, figs, pomegranates, olives, and dates.
The Levush maintains that it is obvious that a lower-level beracha is insufficient for a food that requires a higher-level one. Additionally, a higher-level beracha does not cover foods that call for lesser praise, because an exaggerated beracha is not valid. Thus, for example, reciting Birkat HaMazon for fruits and vegetables, as if they constituted a meal, is invalid, and Borei Nefashot must still be said afterwards.
Two exceptions to this rule are dates and wine. The gemara[5] states that if one recited Birkat HaMazon after eating dates, he has fulfilled his obligation of reciting a beracha acharona, because dates are particularly filling. Another gemara[6] states similarly that wine is filling and would have required Birkat HaMazon if not for the fact that people rarely make it the basis of a meal. Accordingly, the Shulchan Aruch[7] rules that Birkat HaMazon is valid b’di’eved for dates and wine. However, all other foods that require Me’ein Shalosh are not exempted by a Birkat HaMazon that was recited on them outside the framework of a meal with bread.[8]
What if one mistakenly recited Al HaMichya instead of Al HaEtz (or Al HaGefen)? The Levush[9] assumes that regarding dates and wine, if Birkat HaMazon is not too much of an exaggeration, then certainly Al HaMichya is not either, and one would not have to repeat Me’ein Shalosh. However, the Taz[10] disagrees. He argues that Birkat HaMazon includes the word “zan” (roughly, sustain), which is appropriate for dates and wine, whereas “michya” (roughly, food that gives life) refers to a different quality of a food, which does not apply to them. The Eliya Rabba[11] further raises the possibility that the fact that the halacha of fulfilling the beracha on dates with the wrong beracha acharona was said specifically with regard to Birkat HaMazon implies that Al HaMichya is invalid even after-the-fact. However, the majority of Acharonim assume that if one recited Al HaMichya for dates or wine, he does not need to make another beracha acharona.[12] Since the general rule is that when in doubt, one does not make a beracha, this is certainly the proper ruling to adopt here.
The question of Al HaMichya sufficing for dates and wine is much more complicated when one ate both grains and dates or wine, and he said Al HaMichya without also mentioning the other element. In that case, we may assume that the person, in omitting the other elements, demonstrated that he did not remember the need to have the beracha cover them. While there are differing opinions on this matter,[13] the stronger view in that case is to repeat Me’ein Shalosh with just the missing element.[14]
The clear consensus is that one does not fulfill his beracha acharona obligation on grapes, figs, pomegranates, and olives with Birkat HaMazon[15] or Al HaMichya.[16] Therefore, one should repeat the beracha acharona with the correct text.