There is a conan-exiles-legacy branch on Steam, If you plan to stay on UE4 you should switch to it now.
If you want to have UE4 and UE5 versions at the same time, you should switch to the Legacy branch now and make a copy of your /steamapps/common/Conan Exiles folder somewhere else to prevent it from being updated and then switch back to the Public branch in Steam so it can update to UE5.
To change the branch, Right-click on Conan Exiles in your steam library, click on Properties then click on Game Versions and Betas. select conan-exiles-legacy.
Needing a copy of the UE4 version is speculation at this point, but it is how you have multiple versions of other steam games. making a local copy is faster than downloading 100GB again later 
