You can change only home games.
I believe that having your players on the best position they perform is the most immportant thing.
Took me a while to decide where exactlly to play every single MC and DC in my team.. they been getting rates from 5-8 while i was testing thoes.. once i figured it out they started getting 7-9..ya i know its weird but thats how it works for my team.
For example i had 2 guys with AML/AMC/AMR ability and if i switched their places they would perform bad and never score..if they were in the correct positions they would score alot more somehow..
I recentlly bought 19y.o MC with 5stars 1skill points to 6*..Started using him as central midlefier and he was getting rates between 5 and 7, switched his position on the left side(MC) and he started getting 7-9 somehow..