[ESMA 70-872942901-38 MiFID II MiFIR market structures Q&A, Q&A 7.1]

Yes, the authorisations granted under MiFID I should continue to be valid after 3 January 2018. This applies also to the authorisations for market operators to operate an MTF. However, ESMA notes that competent authorities, as set out in Article 44 of MiFID II (and, already, Article 36 of MiFID I), should monitor that market operators comply at all times with the conditions for initial authorisation and therefore regularly review conditions for initial authorisation.