Can I change registration rules of Champions League?

[Mihailo]

I want to remove the restriction to the number of players (currently 25). Can I change it using advanced rules editing?

[krakonico]

If I recall when you change the competition (adding it by yourself) you have to create the rules from scratch, so if you want to remove that restriction just don't add it yourself, but if instead you are editing another person's work, or a previous file done by yourself, just go to where that rule is and edit it as you wish.

[Mihailo]

i can't find in advanced support where it is to change the number of registered players

[krakonico]

Just now, Mihailo said:

i can't find in advanced support where it is to change the number of registered players

So your case is to do it.  Well, is a hard work, because you will need to create the whole competition.

Go to the menu "Rules", then to "Add Continental Rules", then the "Convert to Advance Rules" will appear.  In your case, I don't recommend you to do so.  I recommend you to ask for it, or go to @Carlito85, he has lot of Champion Leagues files, and maybe one of them will help you, even he can edit one of his files in order to let you get ride of that rule.