windows.old is just for undoing the update, if you do;t want to keep using the new version. A very good thing, but once you are satisfied that the new version is good, you can safely remove it and free up the disk space.
Windows 10 is very good at remembering previous monitor setups. This works out very well for me, as I use my laptop in one of three ways. In the office, it's connected to two external monitors, the laptop lid is closed. Out of the office, I am either operating the laptop alone, no extra displays, at 150% magnification so I can actually read things on the screen (14" is too small for 1080, sorry), or I have my USB external plugged in because I can't work productively for long with just a single display. Prior to Windows 10, I was constantly having to rearrange my desktop icons if I scaled the display, or it would put the displays int he wrong order, where moving the mouse to the right woudl start ont he rght-hand screen and then flip to the left hand display, so I would have to go into the display settings and reposition them. This doesn;t happen any more, it comes right up in the settings matching the configuration I am plugged in to.