Hello Author. I set the world border to 30000 blocks wide for now and I pre-generated chunks. If I'd like to expand the border to 50000 blocks wide, when I start pre-generating, will Chunky load the chunks that have been loaded before?

No, as mentioned on the FAQ it will not load them again, but it will scan over the chunks quickly to determine if they were already generated or not. This is quick, but may still take a few minutes.