This is more complicated than that. First, you have alternative railroads, so while Ukrainians can (and will) make logistics harder, the won't be a big problem (seized munitions and fuel is though). Another issue are power plants.

They do have reserve troups. The main issue is that those cannot be redeployed that fast, the redeployment will show weakness and be costly, at least politically.

Also, they have to move their S400 first (which are easier to move than patriot system to be fair, but will have to be recalibrated). Which is why the Falcons are a huge deal and their arrival probably unlocked the offensive (depending on their payload they could also be useful as more than a threat).