Yes, DOWNSHIFT is a Scrabble word!

Letter values in Scrabble US: D 2, O 1, W 4, N 1, S 1, H 4, I 1, F 4, T 1.

Is it a valid word?

DOWNSHIFT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a change from a financially rewarding but stressful career to a less well paid but more fulfilling one
a change to a lower gear in a car or bicycle