Evaluating the fairness of electoral reforms is a critical requirement for electoral accountability in any democracy. There is, however, no research measuring the expected seat benefit for incumbent reformers in newly democratized countries. Much of this delay is due to the technical difficulties of generalizing previous seat-vote models to multiparty races, a problem that has limited both subnational and cross-national comparisons of electoral regime change. Using a multilevel Bayesian model we solve this analytical problem and produce comparable estimates of partisan bias and majoritarian bias across the Argentine provinces. Our model estimates the effect of reforms across many electoral regimes and can be applied to comparative analyses of electoral reforms within and across countries. In the particular case of Argentina, we show large seat premiums for incumbent parties initiating electoral reforms.