Again, i'd draw a comparison to Fleetwood Mac, started off entrenched in the blues, became more rock, then more pop, then a bit experimental, then kind of had a country influence with Bekka & Billy in the band (highly under-rated duo in their own right, really loved what they did with the band, shame it never lasted...)
ABBA started off as Swedish folk, became more pop, a bit more adult contemporary near the end, Benny & Bjorn then went into musicals... a whole range of talent, and cross-over appeal there...
Niv