You need to give the complete sentences you want to say or thoughts you want to express in order to get the Dovahzul translation, as there isn't a word-for-word translation for "choose" or "change". However, here are some suggestions:

"Change to" is equivalent to "become" (meyz or kos) -- "By fire ice changes to a river" -> "Naal yol iiz meyz rath" (By fire ice becomes a river)

"I choose to fight" will be reworded to "I will fight" (Zu'u fen krif).

If you reaaaaalllly want to express choice as in choosing from one thing or the other, you can also use miraad (door): "There are many choices here" -> "Pogaan miraadde het" ({there are} many doors here) -- Source: Odahviing in ES:V talks about choice relating to doors.