What we understand from the commentaries, explanations of acharyas is that where the chariot had now reached, this was kind of border of Mathura – Vrindavan border and Krishna and Balarama, they are going to enter. But the Vrindavan Krishna, Vrindavan Balarama, they are not going to enter.
“padam ekam na gacchati vrindavanam parityajam”
Leaving Vrindavan behind, they don’t keep even one step outside Vrindavan, that’s Krishna. So Krishna is transforming himself, He is becoming Vasudev Krishna, Bala