One can change all of the instances that use a particular parameter set to a new parameter set by changing the parameters of the sub-master cell of the instances. The original pcell must be accessible, as for changing individual instances. The procedure is to edit the parameters of a sub-master, which will have the effect of reparameterizing all of its instances.
A quick way to do this is to select an instance of the sub-master to be edited, and press the Push button in the Cells Menu. The editing context will be pushed to the sub-master. The sub-master can also be selected for editing from the Cells Listing pop-up (Cells Menu), or by giving its name in the Open command (File menu).
With the sub-master as the current cell, bring up the Cell Property Editor with the Cell Properties button in the Edit Menu. The listing of properties will include a line for the pc_params property. Select the property by clicking on it, then press the Edit button. Again, the Parameters pop-up will appear. One should modify the parameters desired, then press Apply. The new parameter set will then apply to the instance pushed into, and all other instances of the same sub-master. Use the Pop button in the Cells Menu to return to the original editing context if Push was used.