Everything on the title + screenshot

https://i.imgur.com/QpVWtrN.png

This is a known limitation. Was upgrading possible in earlier versions? Which version(s)? Fast-replacement got disable in 0.0.7 according to the changelog, and I think this means that upgrading was also disabled then.

I hope to fix it in version 0.1.0, but currently 0.1.0 allows connecting different levels of linked belts via up/downgrading which I would like to fix. Let me know if you are willing to allow that if it lets you upgrade belts.

i don't really understand what you talk about at the end, i'm not english ^^'

That's fine, I'll rephrase it.
I'm working on another update for the mod, where upgrading the linked belts is possible, but the simple way of allowing players to do this also makes it possible to have a Linked belt connected to a Fast linked belt. I am still trying to fix this to make it work like underground belts.