If  I wish, can I mid-play kill units (e.g. reduce 2 units to 1)? Or does discarding units HAVE to be forced into play by supply limits being exceeded by such events as retreating, adjusting supply, etc.

Example:
I'm at step 5 on the supply track (4|3|2|2). I have already filled up my max quota of 4 armies in total as follows: 4|2|2|2. Now I desperately want to split my army of 4 units into two smaller ones. Can I now, mid-play, reduce one of my 2-man-armies on the map to 1 unit, then proceed with my march order for the 4-man-army and do the split?

(P.S. I would presume that discarding units - without being forced to do so - is not allowed. The only thing I found about it was in the case of retreating and getting in conflict with supply limits, you have to choose units from the retreating army to discard. If you can't randomly discard units from another area when retreating, I presume you can't randomly discard units at any time. Just want to double check with you experts to be sure.)