Italy changed its electoral law in 2017, with a simple majority vote in the parliament. It strikes me as peculiar and dangerous that it is relatively easy for the ruling party to change "the rules of the game" in a fundamental matter. It looks like this possibility can easily be abused, for instance by altering thresholds to enforce a two-party system and effectively remove many competitors from the race.
How easy it is to change fundamentally the basic electoral laws in other countries? Let us refer to the major Western democracies, for instance, to keep the question more focused. For instance, to turn a proportional system into a majority first-past-the-post system? Is the electoral system usually defined in the constitution? Does the change require a popular vote?
I know about gerrymandering, but I am more interested in the possibility of altering the rules on a more basic level, not merely redesigning districts.