hi alexias, i think the aspirated consonants changed to fricatives a few hunderd years AD. in BC, theta, phi and chi were definitely pronounced as aspirated consonants: they discovered this looking at inscriptions in other languages, e.g. latin, where they wrote the greek word for "philip" with a P or a PH, rather than F, which would have been the right letter in latin if the greeks pronounced the start of philip with a fricative.
incidentally they determined some latin consonants by their transliteration in greek, e.g. V in latin... the greeks used the diphthong "ou" rather than something like beta.
the book to read is "vox graeca". it pulls together all of the evidence. but choose whether you want to follow his recommendations
, which seem to me to be basically designed for people who have been pronouncing greek incorrectly for years (e.g. pronouncing phi as a fricative rather than an apirated plosive). he often recommends that you follow the old incorrect techniques for consistency, e.g. using a stress rather than pitch accent, things like that. cheers