Hi Faro,
If AR_par is the parameter that regulates the persistence of the shock, and in your benchmark calibration you have calibrated it to 0.8, what you can do is to use set_param_value as in the following minimal working example, where I plot the impact the shock has on inflation under the two different persistence regimes.
// model block
// shock block
stoch_simul(irf=20,nograph);
large_persistence = oo_.irfs;
set_param_value('AR_par',0.1);
stoch_simul(irf=20,nograph);
small_persistence = oo_.irfs;
L=1:options_.irf;
figure
plot(L,large_persistence.pi_eps,'-b',L,small_persistence.pi_eps,'-r')
For more complicated plotting, see for example