Starting with Appendix 3 to Annex 1 to Part.FCL for the benefit of others "An applicant wishing to transfer to another ATO during a training course shall apply to the competent authority for a formal assessment of the further hours of training required." However in your case as you have done no flight training yet it shouldn't be a big issue, you obv won't get any hours credits. If you want to stay in Greece the best three are regarded as Global Aviation, Egnatia, and Superior Air. I know the first two quite well, Superior Air less so, however it seems likely that one of these is likely to be your current ATO so I'm not entirely sure how to advise you. If you want to go outside Greece where do you want to go? Would you consider modular training?