no, Resellers cannot edit the packages, you have to your system adminstrator(Host) do it for you, in their Root WHM.
I believe the reason is because when you change a package, anyone that has that package, will be changed to the new changes, so therefore that would be like upgrading/downgrading an account, and if your a reseller that option has to be given to you.
They would have to add a check to see if you where given that right, and if so, I guess they could have it add that to the WHM,
I personally don't allow upgrades because of how I cannot charge them until after it's done, and I want to make sure its all paid for before doing it.
Things should be set up so that a reseller can only use the packages that he has defined for his own customers. That way there is no hrm in a given seller updating his own packages. [Although that may not be legal in certain states! ]