Variable switching to constant in DSGE

I’m sorry, this question might be strange, I am new to DSGE.

I am trying to model a housing mortgage program in DSGE. Basically, mortgage interest rate varies until the government sets it to a certain rate (very low compared to the policy rate). Even though the mortgage rate does not change after that, other interest rates can vary.

Please give me advice on how to estimate the effect of this change. Please share any ideas without hesitation.

You need to specify the information structure here. Are agents aware of the regime switch? Do they anticipate it? Do they know whether the change is permanent. Do they anticipate that there is a probability of switching back?