What happens if a lane goes down or becomes faulty?
If it is lane 0 that is lost, the link goes down and dies.
If it is lane 1...n (any or all of them) the link goes into what is called "recovery" and will attempt to recover the largest viable link with whatever lanes are still operational.
So, for our x8 core, the loss of lane 1 will yield a recovery to x1, where the loss of lane 6 would yield a recovery to x4.
Once this happens, if the failed lane were to become alive again, the core will not attempt to recover to a wider link width. The only way a wider link width will occur is if the link actually goes down and it attempts to retrain from scratch.