If you are going to the trouble I would do them all, I lost several in my V6 including gas and battery over time. I had found a speedo shop on ebay that would do the whole swap out for a decent $.
Traded: Legend Lime V6 5 speed @ 199K miles
Now Driving: 2012 Red Candy GT Premium 6 speed MT used with 59K miles. Steeda tuned. TSW Nurburgring 19 x 9.5's w/Cooper Zeon AS. 55D springs & Koni Strt.