With signs that Montreal's more than decadelong condominium boom could be fading, some local developers are repositioning or even pulling projects due to waning demand.

In the downtown core, quarterly presales of new condos have averaged nine units per project this year, according to Altus Group Ltd., a real-estate consultancy. That is well below the pace of such sales in both 2012 and 2011, when the average was 16 units.

Meanwhile,...