So it's highly possible that there are more elegant ways to do this and I may be noobing for all to see here... but I hope nobody out there is depriving themselves of replaying the mixed-jersey ASG's (97 to 02) if that's what their heart desires.
So please note the following, and if this helps one person then my job is done.
1) In MyLeague, if you change the Rising Stars game to Rookies vs. Sophomores during the offseason, it will now play with mixed original team uniforms.
2) You can also change the teams to 12 players, can't you?
3) If you have Hook and Tools, you have full control over era-specific uniforms, don't you?
4) You can make any player a rookie or sophomore by changing Years Pro to 1 (Rooks/home) or 2 (Sophs/away) can't you?
5) From there, you just need to grind through each roster and filter out any unwanted rooks and sophs by changing their injury status (I use twisted knee) until you have your desired roster. ***Always backup your ML before you hit Play Game; if you overdo it and go below 12 of each, it won't play.)
A court mod here... A jersey mod there.... and suddenly you have something like this on your humble current-gen version.
All for the low, low cost of a scoreboard that still says Rookies/Sophomores.
And remember: you can insert any player you want by quickly importing their DNA into a Create and then copying them into the FA pool.
But this is probably old news.