It can also mean those units are not manned at full size. For example, if they have half as many pilots and crews, then those pilots that are present in the unit can still get some half decent 100-ish hours per year of flight time, even if they're flying half of those hours on one airframe and other half on another airframe. That way those very old planes could still possibly be kept airworthy, since they're not flown very much. Indeed, looking at the satellite images of such J7 bases over time does give an impression there's rarely more than several of those around the airbase.
The downside would be that the unit is still kept under the numbers of personnel needed to operate a full squadron of modern planes. So when and if they do receive J10s, for example, they still might need to expand a bit, personnel wise. Of course, adding 30 or 50% more personnel is still easier than having to start up an airbase from scratch and stand up an unit from scratch.