it will be awful. The plot stops making sense, the characterization of the characters that we love and has been spot on so far starts to deteriorate, and many other things. It'll feel like a different writer.
With that said, if you can endure it, the story does get a lot better afterwards. Not as good as it's been so far (it never quite recovers and the author forgets things they've previously written, replacing it with inferior solutions... lots of plot holes), but if the story now is a 9, next arc falls to a 2, then the series bounces between a 7 and a 4 going forward after the next arc.
It'll still be a fun story, that much I can assure you.
The possible good news: if the mangaka here is smart, they'll fix the plot holes and the writing and be able to do so without changing the plot points of the story. This means, we could see the story reman as good as it is now.
One example of this LN author's forgetfulness (that DOES NOT SPOIL ANYTHING): there's an issue with security next arc (that is briefly mentioned once and has nothing to do with the plot or characters). The (logical) solution would be for the guards to set their guns to stun (as the main character has done so in the first couple chapters). The LN author forgot about stun mode and made a scene absolutely ridiculous and nonsensical that's played for laughs (you won't guess what it is from this because I didn't specify anything). The mangaka could just remember what the LN author forgot and change the result to get an equally funny scene that makes way more sense.
Here's hoping for the best.
