They could convert the existing buildings too, it'd take new cables and some inside work too. I guess a dual-voltage transformer wouldn't be enough for running both systems in parallel since the old setup requires a floating star point (IT) while the new one needs an earthed one (TN).
I suppose they could convert the older parts to 133/230 V TT if they're currently 133/230 IT with earthed local PE. That'd be the Belgian system, once common in quite a few European countries. They'd still need a transformer with separate secondaries I guess. If all existing supplies are single phase (i.e. only two out of the three phases are supplied, giving 230 V) they could replace the distribution and reconnect the existing buildings to 230 V line to neutral instead.