After halting ngspice by stopping the background thread (19.4.2), nearly all ngspice commands are available. Especially alter (17.5.3) and altermod (17.5.4) may be used to change device or model parameters. After the modification, the simulation may be resumed immediately. Changes to a circuit netlist, however, are not possible. You would need to load a complete new netlist (19.4.1) and restart the simulation from the beginning.