What will happen to contacts who were a part of the old flows

Hi @nayanisakina99, welcome to the community!

There’s a few ways you can approach this. You can either add some Flow Filter conditions relative to a Date so new folks after the Date won’t enter the Flow. 

Or as @KatherineB suggests in a previous post, you can add a “Conditional Split” at the very top of the old Flow to stop people from entering the rest of the Flow. 

 See here: 

In either way you can keep all the Flow Messages Live while existing people complete the Flow, and the new profiles will go to the New Flows.  Once all the people in the old Flows have gone through, you can then turn off the Flow.

Hope this helped!