Having to elaborate or modify an NIH grant aim
The answer somewhat depends on the grant mechanism and whether or not it has been funded. If the grant has not been funded yet, do whatever you want. If you are talking about modifying a submitted proposal, you will want to talk to the program officer.

The mechanism also matters. As you are a graduate student, if the grant is an F31 or T32, then you are not technically incharge and you will want to talk to the PI (likely your advisor). If it is truly your grant, then you may want to talk to the NIH program officer. They are your friends and have a good enough understanding of the field to see if something is truly needed and if it would require some sort of amendment.