Using an 8 ohm speaker should not shift the crossover point but it will have less output than a 4 ohm driver. Matching to the mids might be harder since the levels might be off. Depends on what the crossover was specifically designed for.
Thanks grand master the crossovers I have are rated at 600 watt but the ev sub just 300 .. probably should go buy some but these are new and lying around lol