How do my worlds change with the Nether Update?

I play Minecraft (Bedrock Edition) on my phone. I have a really good world there. I have already played a lot with it and also a Nether Portal. I've also found a fortress in the Nether where I always farm XP from Lohen. If the Nether Update appears soon, how will the Nether change in my world? Can I still play in the world at all?

It shouldn't have anything to do with the worlds. The nether has to be deleted and regenerated so that the new items appear. It is not more. However, where you come out when you go through your portal is a matter of luck, you can also fall into the lava. Possibly create a new portal.

Is the nether automatically deleted or do I have to do it somehow?

If you play on a server, you have to delete it yourself, in the single player I don't know.

It's not a server, but I've played with another player in the world

Sorry, I'm not familiar with the single player, I only play on my own server.