Ever since Magic: The Gathering started up in 1993 there have been five colors: red, blue, white, black, and green (and kinda purple, but not really).
Throughout its history they have each had a unique symbol. Unlike, say, the tap symbol, they have never really changed. Blue is a water droplet. Red is a flame. Green is a tree. Black is a skull. And white is a sun -- though it hasn't always been the sun that it is now.
So, this is the current white sun symbol that's been in use for a good quarter century or so:
The shift had been made by Magic in early 1995 due to wanting a more crisper, uniform look and to look more like a sun and less like an ink spot with a ring in it.
However, it actually caused a bit of grief early on. Ice Age had received some previews in early 1995 and needed some cards to show off before full production. Well, the "new" sun wasn’t ready, but the previews were desperately needed ASAP. So Wizards of the Coast decided to split the symbols and have cards with both of them.
Two cards chosen for early previews, Fylgja and Prismatic Ward, went to full production with the old symbol. Luckily for Magic, the symbol was finally ready for the rest of production a few months later in time for Ice Age’s release in June of 1995.
One little change ended up making Ice Age the only set with such a switched symbol set, a final decision that left Magic “unhappy”.
While no color symbol has changed since (not significantly, that is), future symbol changes, such as the aforementioned tap symbol, made sure to have it done and ready well beforehand. And for any future symbol changes, you can bet they’ll do the same and not create another mixed-symbol set.