you don't have to be scared about opening the screen again.

If you have 5 units of 40 hastati (I play on small) and 1 unit of hastati with 1 man, place the one man unit in the retraining queue, merge a 40 man unit into the 1 man unit and place the new one man unit into the queue etc,

you end up with 5 units of 79 hastati and 1 unit of 40 hastati in 1 turn.

do this for newly conquered cities will generate uber sized units in no time and also rapidly deplete the population so you can push on faster.