It's always better specify what you will do before the initiative, specially for the spellcasters, since the spellcaster actually needs a foul round to cast a spell. So I suppose from the end of his turn till the beggining of his next turn the spellcaster actually starts casting the spell which is finished by the end of his turn. For this reason he rolls a concentration check if he gets any damage in order to keep his concentration and don't loose the spell.
OTOH the fighter can decide to change target if he wants.