If the original speaker was 4 ohms and the one you have replaced it with is 8 ohms, the amplifier's output power will be roughly halved, so you will be less likely to blow the soundboard's amp as it wont be working as hard to run the 8 ohm speaker.

Its only if you go LOWER in impedance (eg replacing an 8 ohm speaker with a 4 ohm) you're asking for trouble   In that situation the amp tries to put out more power than its capable of which can, and usually will blow the amp, especially if the volume is turned up loud!!

the box was marked 8 ohm(2 way), so does this mean each spkr is appx 4 ohms?(as i using only 1 of 2 spkr)

No each speaker will be 8 ohms, there would be some sort of crossover in the hi fi speaker box so the amp its hooked to would only see an 8 ohm load