FSM and SYRIZA are the main anti-establishment parties which seized power during the Great Recession. As in the Greek case in 2015, when SYRIZA coalesced with right-wing party ANEL, FSM coalesced with a radical-right party (The League). Regardless of their different ideological backgrounds, both SYRIZA and FSM were relatively ‘new’ parties at the time of their first relevant electoral performance. While the literature has so far tackled the issue of the growth of these two parties in their political system, their organizations and their electorates, little has been said in comparative perspective on the internal reforms that the two parties undertook and their institutionalization process. The aim of this paper is to enquire into the nature of the party reforms within those parties, which the literature has labelled as anti-establishment or populist. To what extent do their reforms correspond to the theoretical frameworks that were designed in the past for mainstream parties? And, secondly, are these changes that the parties have undergone similar? In what ways? The findings show that, albeit following different paths, party changes in the two parties followed the same pattern as mainstream parties.