Does upgrading a white ancestral item keep its item level?

Yes, that tip is basically the point. If the base item is ancestral, the upgrade path uses that base item level instead of resetting you to something weaker. So if you are fishing for a good unique outcome from a white item, the item level on the base matters a lot.

That is why people pay attention to the highest power white bases they can get, especially from random purchases. A low base can still turn into the right item, but the stats you end up with are usually better when the ancestral base is already strong.

The practical trick is to treat white ancestral items like hidden starters, not junk. If you are using a system that can upgrade or transform them into a unique, you want the cleanest high-item-level base you can find before you commit materials.

That is especially useful when farming from currency-based shops, since those can spit out a lot of low quality noise. Checking the item power first saves you from burning resources on bases that were never going to give you the better version.

Yep, and the reason people mention this is because the upgrade step does not re-roll the underlying item level in a way that helps you. If the game lets the ancestral base carry through, then the starting item power is the thing you should care about.

It is one of those small optimization details that feels minor until you are chasing a specific unique and every bad base costs materials. Once you know it, you stop ignoring white ancestral drops completely.