This might be not the right place to post, but at the moment, me and some friends are playing on a custom server i made for us all. At the moment, its running 1.14.4, and also the terrain has been generated on 1.14.4. As 1.15 versions of spigot are available, i had the idea to upgrade to 1.15, but because of new blocks and stuff like that, im a bit worried they wont generate and even worse, the chunks will get f'ed up. How do i fix this issue ?
That's perfectly fine. Downgrading would be a issue, not upgrading. Before you update the world though, make a backup incase if anything goes wrong. --forceUpgrade in the server arguments.
As a side note, only add --forceUpgrade at the end of the server start line once, otherwise every time you restart your server it will take a while to rejoin.