Depends on your army fluff - if they're supposed to represent termies who've only recently fallen to Chaos, then that could need no more than painted chaos symbols defiling their old loyalist symbols. Some head swaps and weapon swaps would also be good (and in some cases compulsory, as chaos can't use some GK weapons)
Over the centuries, one would expect them to slowly mutate into something more like the standard chaos termies, i.e. much spikier, and with no trace of their original imperial loyalty. This would be a whole lot of work to convert, unless you're a brilliant greenstuff sculptor.
Hmm, if you want a Tzeentch theme, i suppose you could go the other direction fluffwise - say that they're chaos termies who've been newly mutated into a form mimicking GK to mock their loyalist enemies. But to do well, you'd still need some conversion (to stop them looking like ordinary GK models repainted) and the weapon swaps as mentioned before
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