I'd like to suggest to add the same skill upgrade chart we all have on our own players with all abilities upgrades/downgrades and weeks that has passed, to u21 and nt teams.
Most of NT coaches have some sort of spreadsheet but this would be a good addition to keep a better track of palyers, even if you select someone new, you could see their previous upgrades.
I don't think there is anything else to explain. Pretty simple, help a bit more those managers who spare some of their free time to nt teams.