I watched a video demo of Terry Bell going through the different tunings on a minstrel banjo. It was playable immediately after he returned.
My experience with nylon strings on my banjo has been quite a bit different. After I change tunings on my banjo it has taken hours to days to hold and be playable again.
Am I doing something wrong?
What type/brand of nylon strings are you using?
I use Aquila Nylgut 'Minstrel' set (a little heavier gauge than Nylgut's 'Classic' set) and after the first few days of breaking in, they seem pretty stable when I change tunings.
That said, any time you change tunings the strings will exhibit a tendency to creep back in its previous pitch's direction- but that minor and after a half a tune you can tweak it a bit again and it should be fine.