In general, what Kevin said. There is no reason to replace them just because they are not a matched set. But, there are other reasons you may want to replace them...
Do you know how many miles on those tires? I assume they came with the bike, so maybe you can't quite tell. A couple of things to check other than whether they are worn enough to warrant replacement is their manufacturing date. Depending on the mileage, it's possible that the front has not been replaced in a long time (fronts often last much longer than rears). It may look like it's still within the wear limits, but the rubber could be old and not as grippy as it was intended to be. That would be a good reason to replace both.
Also, I think Rob meant Continental TrailAttack 2 and 3 (senior moment...?
). The TA3 in particular is a very good tire, significantly improved in wet grip over the TA2, so given about equal price, go for TA3s. Pirelli Scorpion Trail II are also a good bet, even a little better than the TA3 in the wet.