I don't really feel as though they changed Syndra's lore, per se, more as they added to it. They added childhood ridicule and tumultuous imprisonment by the Spirit and nestled Syndra's old lore right in-between.
And, you say it's "change for the sake of change" - but I feel as though you're ignoring the depth of character added from it. Syndra's not just "power-hungry magic queen", she's a wrathful mage spurned by her own people for the uncontrollable malice and magical aptitude she holds - and she's still Syndra. Holding loyalty to no-one but herself.
If you're going to argue against changes or updates, at least do so for the right reasons. Syndra's lore maintains the core of who Syndra is - a former Ionian mage who was at one point suppressed, and now unbound to use her power as she sees fit.
Karma, on the other hand, wasn't quite so lucky for a time. With her initial lore update (with Irelia's VGU), she lost the core of her being - the spiritual leader that encourages her people to fight when necessary. She became a furtive recluse after an accidental display of power. Fans of Karma and the lore in general were rightfully disappointed with that - and it got fixed.
Syndra is still Syndra - just that now, she has a bit more depth of character to her. It hasn't changed who she is now to show us who she was in the past.
You guys even retcon lore that wasn't even a year old!(darkin)
...Aatrox was released in 2013, and the major Darkin lore update happened four/five years later.
The Darkin lore was definitely more than a year old.
