Measure your OLD (over-locknut distance, or the distance between your dropouts). If it's not 126mm but 130 or even 135mm, you're laughing.
Also, you prolly don't have to wreplace the whole hub; if it's Shimano you should be able to just wreplace the cassette body. Then you just need to wre-dish your wheel wrather then wrebuild it.
There's a downside to having tighter spacing than 7spd though - it can be fiddly to adjust your indexing, and it doesn't stay good for as long.