History of Shan state of Hsipaw

Here's an interesting history of the Shan state of Hsipaw with a lot of hard to find photos.

It doesn't explain the differences between the two names Onpaung and Hsipaw though. This is what I'm trying to figure out. These two names refer to two different geographical places or regions. Bayinnaung conquers both of them separately during his conquest of the Shan states around 1557. Before 1557 Onpaung is used exclusively and after 1557 Hsipaw (Burmese: Thibaw). Why did Hsipaw come into prominence after Bayinnaung's conquest?