Change of position at the company: do they have to raise my salary?

Don't assume they'll automatically raise your salary just because you're changing departments or positions. What really matters is finding out what the standard salary is for that role at your company and in the market - that's where your negotiating power comes from. I changed positions a while back and just accepted what they offered without doing any research, then I found out that people in the same position were making quite a bit more, so next time I came more prepared and got what I deserved. If the new role has more responsibilities or requires more experience, you have every right to ask for it, but bring concrete data to the conversation.

What usually happens in practice is that companies keep your salary the same if the change is horizontal (same hierarchical level), but if you're actually moving up in rank they should offer you an adjustment. I had something similar happen at a place where I worked and I had to explicitly ask for a raise before accepting the new position, because if you don't bring it up right then it's way harder to negotiate it later. The key is knowing beforehand how much someone makes in that role at your company and outside of it, because that gives you solid arguments for the conversation with HR.

Before you accept the new position, explicitly ask what your salary will be after the change - don't take anything for granted. I made that mistake a while back when they offered me to coordinate a bigger project at my workplace. I assumed it would come with a raise, but it turned out the company was planning to keep me at the same level. I ended up negotiating later and it was way more awkward than if I'd cleared it up from the start. If it's a real promotion (more responsibilities, more people under you), it makes total sense to ask for an adjustment. If it's a lateral move within the same rank, that's grayer, but you can still make a case if the new role demands more skills or workload.

The reality is that yes, they can (and often do) without being legally obligated, but that doesn't mean you should accept it without pushing back. What the other comments don't mention is that your bargaining power is at its peak *right when they offer you the position* - once you accept, it's much harder to ask for raises later. So yes, find out the market salaries for that role, but also keep in mind that if the change involves more responsibility, longer hours, or a higher position level, it's completely legitimate for you to demand an adjustment, even if it's modest, before you say yes.