The positional bump alone won't be a massive change to his defensive value, but the accumulated value of his defensive contributions (at least FWAR wise) are likely going to see a massive bump. A large part of why his defensive FWAR is so low is due to the fact that Fangraphs uses Statcast OAA for their range factor, and Varsho has posted rather ordinary numbers in left field according to this metric. Shift him full time to center field and the range value is almost certain to receive a massive increase on top of the positional adjustment.