I have an issue with the world I just finished creating.

To start with - a long time ago I created a world and installed it, I'm still playing it, but I wanted to make it better. So I used it's file and simply started making corrections, adding more stuff/removing some etc. I re-named the world I was working on now and even gave it a new picture. I exported it and installed. I was shocked to see after opening game that the previous world is just gone (not showing up in launcher) , while this new world is installed I cannot install old one again saying "it's already installed" - but invisible and saves using this one cannot be played ("missing files"). After deinstalling new world I can reinstall old world just fine and play again.

I really spent a long time perfecting this world. I would love to keep both worlds in my game. All help is appreciated <3

You will have to change the world ID so that both are unique worlds. You will have to start a new save though, but that is true in any case.

This is how you change the ID. Mind, I am talking about the CAW files here, not the exported world: Back up the world-file (i.e. just make a copy of it somewhere). I never had any problems, but just in case. Then open your .world-file in S3PE and find the resource file with the tag WPID. Delete this resource and save (just save, do not save as). Close s3pe.
You won’t notice any difference but next time you open save your world in CAW, CAW will generate a new WPID resource. And that’s it.

Thanks, it worked <3 I knew I had to do something using s3pe, but I had no idea what should be done to the file. I owe you one <3

So it's resolved