Why does updating a resource assignment not affect the cost plan?

[SimonJ473792863]

If we alter a project resource assignment with a revised effort and recalculate costs we see the expected adjustment. If we change a role to a different one (which has a different rate) and recalculate, we do not see a cost adjustment. If we do both of these things we do see a change to cost and using the updated rate as we would expect.
Why would changing the role alone not apply the new rate and adjust the cost?

[JayanthiHCL]

cost are generated based upon the primary attributes, also the defined rate model such as Group, role, user per your org. 

[SimonJ473792863]

We already have role as a primary attribute and have rates for them.