Update the Grade on the Position Record

We are changing the names of our rates and have a process in place for getting them updated on the assignment, so now we need to find a way to update them on the position record. 

There are several hundred of these to update so an automated solution would be most preferred.  I am thinking the use of an API or Web ADI import if we can.

Can someone shed any light on which API that I want to use?  I think it is this one but I am unclear as to the field/variable that I need to update.