The evolution of supernova remnants in preexisting cavities is examined by means of 1D and 2D numerical studies of the phenomenon and the fragmentation of ejecta. The analytical approach involves approximating the cavity produced by an adiabatic blast with a power-law density distribution. The numerical studies that follow model a single supernova inside a 'normal' bubble with both fragmented and unfragmented ejecta. A standard supernova-remnant evolutionary path cannot be described because the initial conditions affect the thermal conditions that follow ejecta thermalization. The evolutionary appearance of supernova remnants is found to be highly dependent on the existence of a massive wind-driven shell. The results of the numerical models are concluded to be an appropriate framework within which to study the supernova remnants with observational data.