Updating this as best I can for now but a couple key questions will need to be answered later when better info is available:
1) What does this mean for TH375? Is it extended up old TH1 or is it orphaned? The official map doesn't show the new alignment in this spot yet
2) Further east the old alignment is marked on the official map as TH370. Will need to determine later if this is signed as such, no imagery new enough to show the new alignment is available on Google or Mapillary. For now leaving it out, though a different section of old TH1 further west IS signed as TH370 so it may end up being.
It does also look like they plan to bypass Selfoss itself next so will need to keep an eye open for that.