Scenario:

You create a trenline alert

The alert fires after price crosses the trendline

You move the trendline, angle it further up let's say

Question:

Do you have to create a new alert for that trendline, or will the alert update with the new angle / position of the trendline?

You need to create a new Trendline Alert. The position of the existing Trendline Alert was fixed when it was created.