This morning, when I woke up, I still had a headache. Took some more painkillers and thought about what my Huel intake would be for the day. In the end, I decided that I would skip Huel completely for today, mainly to make sure my headache would go away quickly. I didn't want to provoke it.
I'm not giving up though. It's just become obvious that my body sees this as a big change, and therefore I need to ease slowly into it. I really wasn't expecting headaches to be a side effect.
The headache did pass during the day. Tomorrow I will replace just one of my meals with Huel, and see if the headache returns. If it stays away, I think I will stick with one Huel meal per day for at least a week and see how my body adjusts.
Still trying to decide if I will replace breakfast or lunch, but whichever I decide, I'll definitely try to keep my coffee intake the same as usual, in case that was the cause of the headaches.