As long as the tyres are of the same size and rating then generally it's not too much of a problem, unless the tread patterns are radically different.

What's potentially more of a problem is fitting the new tyres to the front. This may sound like nonesense at first, but the problem is with the greater tread depth of the front tyres in wet weather. Deep treads disperse more water than shallow treads, which means potentially less grip at the rear in the wet. You might think this is a good thing, but not in a panic situtation. Actually it doesn't have to be a panic. I had two new fronts on a Nippa once, and I'd noticed it feeling a little touchy on sharp or fast bends. Then a woman pulled out on me when I was turning right at a roundabout. I didn't need to brake, just lift off. Even so I spun!

I think there's actually a EU standard which states that new tyres should be fitted at the rear. So when you wear your fronts out swap the rears to the front and fit the new tyres to the rear.