Does Garageband update with High Sierra updates
As far as I could see in the HighSierra installer there is no Garageband included in the apps list.
No guarantee, I did not try this:
You could download Garageband before the OS upgrade (in the Purchased list in Appstore), back it up. 
Then "uninstall the actual Garageband in the Applications folder
Then download the HighSierra installer (Appstore Featured).
After updating to HighSierra it should not be there. 
Then you install the backed up Garageband, I am not sure that it will install, but I suppose it does. Then try and see if it runs correctly, do not update.