Minecraft: can you edit a flatland map afterwards?

I built a city in Minecraft on a "super flat" world. Now I have the problem that I would like to build a subway under the city, but with only 4 blocks of the underground it is not easy and not really nice or "realistic".

Therefore, the question of whether it would be somehow possible to move the entire city up by 5-10 blocks and place floor underneath… Etc.

The city is already very big and therefore have no desire to move everything up by hand using copy / paste…

I look forward to your answers!

Have you ever tried World edit?

You can't, but I would recommend that you generate a new world (with the appropriate settings) and then copy the city using a tool like MCEdit.

Make a backup of your world beforehand.

Ah that doesn't sound bad, where can I download MCEdit?
Is this for free?

Not yet, where can I download it?

Yes, it's free, and available here: https://www.mcedit.net/, if you don't get along, you can send the world to me.

For this you need Forge (http://files.minecraftforge.net/) and the WorldEdit mod itself (https://www.curseforge.com/minecraft/mc-mods/worldedit)

First you have to download Forge for the version you want, there's an official website where you can find it. You can also easily download Worldedit from a website

Wow that's super nice of you. I'll try it alone first and if necessary I'll write to you again.