In my opinion, the reasoning behind the change is that high school kids are not the main audience anymore. When I was in high school everybody in my class was watching H&A, nowadays I don't know anyone younger than 25-ish who is watching. If the main audience is older, their characters and storylines have to mirror that. I'm constantly winging on this forum about how bad the writing is. It is not that storylines are repetitive, but the actual writing is so bad, that I feel like the writers are insulting our intelligence.
The fact is that H&A still gets high enough ratings and gets companies to advertise to stay at the 7 pm slot, not to mention that in the last few years they won Logie for best drama, so I don't think that anything will change any time soon - if it's not broken don't fix it.