Found one YAMAHA UX had quite yellowed fronts when I first saw it - I took a few off and sanded and repolished them carefully about 15 years ago. At that time, I realized they were slightly cupped as I sanded them. Looking closer, the surface was slightly crazed. I was at the piano yesterday, the yellowing is now brown (no smokers!!) and the crazing can be seen with the naked eye now, as can the cupping. We won't be worrying about it - it's now 42 yrs old, and really isn't worth a full rebuild - it still plays quite well and my daughter's children enjoy it.
Just lately I've noticed a slight yellowing of the keys on my 30 year old Yamaha used piano that I've had about a year now. The piano receives weak ambient light only, from a nearby window, and I'm particular about making sure my hands are clean before I practice.

Some plastics are more colorfast than others. Many Yamaha uprights from the 1980's have white tops but strongly yellowed fronts. For the fronts only, painting them white is a better option. Some Yamaha's from the late 1980's through early 1990's had keytops that would turn gray. Fairly rare and I don't know the cause of that issue, but for the graying keytops, replacement is the only option.

Any advice on safe ways to whiten them, or products that worked for you is greatly appreciated. There are a few things online, but personal testimonials are better!
You can do wonders with the fronts with a little spray paint. You will spend a little time carefully taping everything off, but a few light or dust coats of a white or off-white spray paint will look better and last for years. It may even disguise some of the cupping simply by drawing less attention to the color difference.

I've painted several sets of Yamaha key fronts. I set a strip of wood under the keys to raise them up a little bit and to stabilize them and then sand the fronts to remove the cupping. Sandpaper on a block of wood works well for this. You can start with a coarse grit and follow with a finer grit.

After they are sanded I place the keys upside down on a table and clamp them tightly together. I tape off the bottom of the keys and the sides that are exposed and spray them with a can of spray paint. Krylon works well.

The ones I've done have lasted a long time. In fact, I've never had to redo any of them.