Changing key mid-piece. The smoothest route is a pivot chord — one triad that belongs to both keys, so the ear accepts it as home in the old key and hears it re-spelled as a function of the new one, then a cadence in the new key seals the change. Keys a fifth apart (one accidental different) share the most chords and modulate most easily; distant keys share few or none and usually need a direct or chromatic shift. Pick two keys and see what they share.
Each chord is named by its role in the old key and again in the new key — the two meanings the pivot hinges between.