You cannot successfully format an 800 Kb floppy to 1.4, nor 1.4 to 800.
False on both counts, I used to do it all the time.
You can turn the 800 into a 1.4 by drilling a second hole. The biggest problem was often the reason they were formatted to 800kb is because at 1.4 they had sector errors. Going for 1.4 to 800kb (or 720k on a PC) was entirely software dependent. I believe DOS used to let you do it without doing anything special. To do the same on a Mac, you had to cover up the second hole with some tape.
I had to do this all the time when the number of disks I owned was limited and the different computers I had access to contained different capacity drives.