Why do Minecraft coordinates change when I copy my world?

I’ve recently decided to find a stronghold on my survival world and I’ve tried it all. Ended eyes and the /locate command have led me nowhere. I’ve read online that stronghold location can change with updates. Since I’m playing on an outdated file, the location had moved. I decided to copy my world to see if I could find it on a newly generated world. The stronghold I found on my new world was exactly where /locate said it would be on my old world, the only problem is that it’s a different place. My question is why do the coordinates change between two worlds that are exact copies of one another and is there a way to find the stronghold in my old world given the coordinates in the new one

Most likely, when you updated your world, the world generation changed. If your old world was in 1.6.4 or earlier, and you made a copy in 1.7.2 or later, it will generate a different world.

From minecraft.gamepedia.com:

Release 1.7.2 redid the world generation entirely, making seeds from 1.2 through 1.6.4 obsolete.

If you want to find strongholds using the old world generation, I would recommmend using Amidst, and selecting the version you created the world with in the "Profile Selector". The coordinates of any strongholds shown should be correct.