remaining units / time

Hello Everyone,

The activity has started on 15th of Feb & the remaining units / time remains at 15 m2/hour until 20th of March and later on it increases to 30 m2/ hour. 

I do that case manually by assigning into resource usage spreadsheet. But i would like to learn if there any special automatic tool that will increase it through activity details. And also using resource curves into the case mentioned before doesn't exactly match with the dates. Like 30 m2/hour after 20th of March. 

Here is my question; how would you increase the units / time after sometime easier? 

Regards,

Errol