We use a closed fist as a sign for needing to use the restroom. Should we switch to the "r r" sign you showed us? I'm concerned about the fact that the first grade teachers will probably be using that sign and I don't want to make it difficult for my students when they move on next year.

I wouldn't worry about it. Next year is a long way away. (Not to mention the fact that they'll have all summer to forget what you taught them. Joke there.)

Go ahead and use the sign you've been using. There's no harm especially if it's been working for you and your students. They'll have plenty of time next year to learn a few new signs.