If the DP6 did not suffer an internal meltdown and you put the power back in the correct polarity the board may still work. But if DP6 is already shorted out (use a meter to check both directions) then it has to be replaced. If your output drivers (ULN2803A or ULN2003A) did not short out as a result of the reverse polarity then the board probably is OK, otherwise you will have to replace the failed output drivers which should be quite easy, since they are socketed.