Usual reason for reclassification is modern techniques of more closely examining plants, and realising they belong in a different class. It's been happening for years - Mahonia used to be classed many years ago with Berberis, because they both have yellow roots - but in later years, it was realised they were not related at all.
I'm not sure how you keep up really - certainly googling a plant name often reveals its reclassification because it tells you the new name too. Otherwise, regular reading of garden publications - and if you're a member of the RHS, then they probably mention any changes.

Thanks, Bamboo. It has always puzzled me.

It's a pain in the butt. The only way that I find out about changes is via the grapevine. Hopefully one of the show judges will be aware of a new name and correct my entry card for me!

I agree, Bulbaholic, plus some of the new names are unlovely indeed - Brachyglottis instead of Senecio being one example. And who do you know who now calls chrysanthemums dendranthema - I don't know anyone who does.

Interesting about the yellow roots. I wonder would that have meant that nettles were in the berberis family??lol

Probably - IDs were based on fairly superficial evidence initially.

the ancients classed things on outward appearance so worms,snakes and eels were called Vermes. Whales and dolphinds were grouped with the fishes as they swam in the sea.

The ongoing classification changes are due to the new scientific methods of identifying familial links. Namely DNA comparison, protein sequences and immunological comparisons [animal classifications]

So it will continue 'for ever' as new methods come to the fore.

Annoying as I dont like change :o(

Thanks, everyone. Very interesting!

Interesting but annoying, I say, its very difficult to keep up and as Seaburngirl says, it'll carry on.

It's constantly happening with fungi too. Very annoying. I had no idea chrysanthemums had been reclassified :-(

Happened some years ago now, while I was doing my training. Everyone still calls them chrysanths anyway, but the problem shows itself if you try to look up Feverfew - used to be a Chrysanthemum, now its reclassified, think its under Tanacetum, can't remember now.