Your threads switch automatically once RCS is enabled for you and the contact you are texting (they also need to have RCS enabled, preferably via Google Messages due to other messages apps not having reliable RCS functionality).
The initial setup of RCS can take quite some time for your device (anywhere from seconds to hours). If you know both of you have RCS active, you can trigger a presence update (e.g. whether a contact supports RCS) by clicking the new thread button and opening your contacts thread via that. That usually unblocks threads and updates their state.
You’ll know when RCS is active via the text input box saying "Text Message • RCS".