Plane with premium seats are always bigger earners than those without if there is demand. Say you can put 8 C seats in for 10 Y seats, that means for the same revenue you would have to get 1.25 times the Y price for a C seat. Since you will get more = $$$.
However, reconfiguring is expensive and the expense can hamstring your growth if you do it at an early stage. The costs take some time to amortize, and you will miss that money dearly especially when you are in competition and get outgrown by your competitor.