Yes, that's the simple way... But it is NOT the right way.
Right now, an entire unit role simply vanishes in the modern era. Modern Siege units do not gain the ability to fire without setup until rocket artillery, I believe two unit tiers later? The best, simplest thing to do is to introduce at LEAST one new ranged unit, move the "no-setup" ability back one unit tier, and combine the two lines that way.
And? Pretty realistic looking at military history. You get aircraft back for that btw.