Did you ever try to mass update assets in JSS?
Even if you like to use the API, you might be more comfortable using Michael Levenick's Tool: JSS MUT.
JSS MUT is quite a simple tool that allows you to mass update Computer or Mobile Device assets' following attributes:
- Asset Tag
- Display Name
- Full Name
- Site by ID
- Site by Name
- Extension Attribute
To use it, download the .csv from JSS, change the values, and reupload it using JSS MUT. The official website has videos that explain the process.
Grab it from the Mac App Store!