Portal 2 spoilers ahoy.
This doesn't seem likely. GLaDOS later reveals that Wheatley was an intelligence dampening core, designed to slow her down by feeding her a constant stream of bad ideas which she would have to process before rejecting. Wheatley's personality does appear to change once he takes over from GLaDOS, but this I suspect is a natural consequence of his stated function.
So, faced with the problem of GLaDOS being out of control, his suggested solution is to replace GLaDOS with Wheatley because this is a really bad idea. Wheatley is not really being evil, just following his programming. Wheatley is presumably completely unaware that he is programmed in this way. After he takes over, Apeture starts falling apart precisely because every idea Wheatley has for fixing the problems in the facility is a bad one (much like the turret crates).
Note especially that Wheatley's initial "help" with progressing through the complex is invariably useless or harmful, which seems to support GLaDOS' claim.
His power craziness post insertion probably comes from a similar place. His programming relies on him not questioning his most stupid and harmful ideas, so it's hardly surprising that his tendencies towards meglomania come to the fore. Also, there seems within the Portal universe to be a strong suggestion that power is a huge corrupting influence on AIs. GLaDOS, for example, is much more bearable in her potato powered incarnation, but with the exception of her treatment of Chell, shows no inclination to rein in her crazed ways once restored to control of the facility.
So to summarise, Wheatley's personality probably doesn't change throughout Portal 2. His actions are rather logical consequences of his initial programming as a "bad idea bear" and the limitations of Aperture's AI technology, which seems to tend to the megalomaniacal in any case.