WHEN will i need to buy a license?

I have found that if a game makes less than 100k in its fiscal year, the developer won’t need to buy a license. But what if my company doesn’t expect it to make that much, so we don’t purchase a license, but then the game breaks that number and earns more than 100k in its fiscal year. Can I purchase a license after it does reaches 100k within it’s fiscal year without facing legal problems?

The upgrade becomes required at the end of the fiscal year, because that’s the way revenue is calculated according to the license conditions. As long as you don’t subscribe to licenses for all your Unity users, you are obligated to stop using Unity. You are still allowed to sell your games, but you are no longer allowed to update them, even when you don’t need the Unity editor to make updates:

During the term of this Agreement, you
expressly acknowledge and agree that
if you are a Unity Personal or Unity
Plus user and the above thresholds are
exceeded, then you may no longer use
that tier of the Unity Software, and
you must either: (a) purchase Unity
Plus (if eligible) or Unity Pro; or
(b) destroy all copies of the Unity
Software in your possession or
control, and cease updating Your
Project Content. Unity will monitor
your compliance with and enforce these
restrictions and requirements
including but not limited to
monitoring the number of downloads of
Your Project Content and any available
revenue estimate data.