From plurality rule to proportional representation

I consider the decision of a parliament that might change the electoral system from the plurality rule to proportional representation for forthcoming elections. Parties are office-motivated. They care about winning and about the share of seats obtained. I examine two different scenarios of how parties in government share the spoils of office: equally or proportionally to their share of seats. If the government is formed by a single party and parties expect that each party will obtain the same share of votes in the next election, the electoral rule will never be changed. Therefore, for a change to occur, the government should be formed by a coalition. I find that a change is more likely to occur when there is a larger number of parties and also when the spoils of office are shared equally among the members in the governing coalition. These results are extended to analyze the decision to change from a less proportional rule to a more proportional one.