Changing a system when cloning a world

I’d like to clone a world I have set up for Old School Essentials. Can I do this and change the new world to 5E?

I’m mainly concerned about retaining scenes and walls etc for the adventure I’ve set up. Tokens aren’t a huge issue as I’d simply grab them from my 5E compendium.

You can’t change the system as it will break many things in Foundry as it expects a specific data format, and Foundry doesn’t let you change the system once the world is created. If you force it (by manually editing the world.json) it could work, or it could make portions of the interface unusable (I know for example, someone could not create any new actors until they deleted their entire actors database).
For what you want to do, I would instead suggest you use a shared compendium and move your scenes to it, then re-import them in the other world.
See : How to share content between worlds

Great. Thanks for that!