The targets of Kitesail Larcenist's enters-the-battlefield ability will lose any other subtypes and card types they previously had and will be only Treasure artifacts as long as Kitesail Larcenist remains on the battlefield. They will retain any supertypes they had.

If Kitesail Larcenist leaves the battlefield before its enters-the-battlefield ability resolves, that ability will have no effect. The target permanents won't become Treasures at all.

If some of the targets, but not all, are illegal as the ability resolves, the illegal targets will be unaffected. Only the remaining legal targets will become Treasures.