My understanding is if you are happy with your existing icons, and the way they look on iOS/macOS 26 - there is no need to change.
I created an icon file for my existing icons and built with the latest Xcode 26.1RC:
5Mb was added to each app (which is significant if your existing app size is only 10Mb),
iOS 18 icons looked wrong (appeared to be using the clear icon for the default),
the end result on iOS/macOS 26 was visually the same as using my pre-existing (non glass) icons.
So no useful benefit.
Topic:
Developer Tools & Services
SubTopic:
Xcode