I've always thought that upgrades in the current form are too restricting. You have to die before selecting a different ability and this doesnt help adapting to the given sutiation.
Upgrades could be selected by using a movement combo (like in mortal combat? xD), or from a GUI menu like now, but you could change it any time you want even without dieing. Some limitations have to be applied of course, like a cooldown between ability swaps.