Can a hero learn new skills in exchange of already learned skills using his experience points?

For example: At the start of the game High Mage Quellen as Geomancer has 3 experience points which he spend to learn Ley Line (1 point) and Stone Tongue (2 points). During the game he summons third stone as the first action (assuming that two stones were summoned in previous turns and are presenting on the map) and attacks according his basic skill Terracall . Then he changes his skills to a new skill Cataclysm (3 points) and as the second action he makes an attack for each stone on the map (three attacks). Next turn he changes Cataclysm to Ley Line and Stone Tongue again to summon three stones. And so on. Is it OK?

As I understand, a hero can change skills any time (not only in the city). Correct?

Unless RtL works way differently than ordinary Descent, I don't think that is allowed. In my understanding, the only reason skills are "changeable" at all is a coding convenience so as to allow for heroes like Arvel or use of items like the Archaic scroll. Without a special ability or item that lets you change skills, you can't change them. The app trusts you not to cheat.

Can a hero learn new skills in exchange of already learned skills using his experience points?

As I understand, a hero can change skills any time (not only in the city). Correct?

nope. can't.

RTL rulebook. p. 12

Yeah from what I remember of the rule book, it forced the player's to make choices when selecting skills. Take some nice skills now, or wait 1 or more points to get a rather nice upgrade. I have always been torn while choosing skills because of this rule lol.