Is it possible to change prices in bulk without rebuilding the whole menu?

Yes. Upload your full sheet with "Add to current menu": every dish already on your menu with the same name in the same category, and not marked Sold out, takes its price, description and photo from the file, and keeps its stock count and its place on the menu. New names are added as new dishes. A lower price set this way counts as a price drop, just like an edit in the menu editor, so if price-drop messages are on, customers who ordered that dish may be told after the waiting period.