Is there a way to morph all larvae at once?

Solution 1:

Yes, it is quite simple once you know how.

1. Put all your hatcheries in a control group.
2. Press that control group to select all your hatcheries
3. Press "s" on your keyboard (that will select all your larve from all your selected hatcheries).
4. Press the kotkey for that unit (ie: "Z" is for zergling, "H" is for hydra, etc.), hold it down, it will then start making a lot of that unit real fast after a small delay. Release when you are satisfied or when you are out of larvae.

To do this though, you need to use the hotkeys and not click on the icons. It is the fastest, less apm consuming way to create units with larvae.