Practicing a formation first?

Hello All, Thanks for the great advice.

I was wondering if it's a good idea for test a formation first in a low stakes (against a weaker opponent) match before trying it out on a big fish.

For example, I have a league match today against a bottom feeder and I'd like to try out a new formation (not radically different from others), but one of my players has a red card -- his backup is injured -- so i cant try it out first.

Then, I have a cup match just one hour later. This cup match is when I want to do the new formation.My team is a bit better than his and i am at home, but he is very motivated.

Is it essential to run the same formation for both matches to get best results? Or can I do something different in the league match then try the new formation? Should I do some training (practice match) with the new formation sometime before the big match? Does training in a certain formation help the players "get used to" the new formation?

thanks a lot top 11 mates ;)