Everything you own swaps over as you just then are playing as the new king rather than your old one. I'm actually about to retire my king before rollover so perfect timing xD.
Once you've selected a new king, your old one will take up the grandpaw slot on the den page, (above the sub male slots), where he will stay till he passes. He can't patrol or anything, but cubs can give some text on playing with him randomly when you go to look at them. I do believe you can still edit his bio and decor as well.
You then continue to play as your chosen king just as you did with your previous one. All your cubs, lionesses, other sub males, items, and currency staying with you :)

Here is the wiki link if you'd like to read it from there as well,
https://www.lioden.wiki/your-lion#retiring

You have liked this post!

Oh okay thank you!

You have liked this post!