You can just switch. Yes, drop the dose, around 30% will put you at just below the Reta equivalent.
While it’s not prudent to run both at the same time, since it takes both about 4 weeks to clear out, and 4 weeks to reach max saturation, as one disappears the other will ramp up by roughly the same amount (Reta will build a little more slowly than Tirz, but don’t be quick to increase the Reta dose if hunger comes back a little. Give it two full weeks before deciding if it’s not enough.