modulate to (some other) key
To transition from one musical key to another. The skilled composer can modulate to a higher or lower key without the listener even noticing.
modulate to a (different) key
to change from one musical key to another by means of a musical transition. Suddenly, the organist modulated to a key that was too high for most of the singers. I will have to modulate to a lower key before I start the next hymn.