One project can only have one project rate per role. Once the project rate is changed, it changes all the historical data as well. We do not support changing project rates by time period at the moment.
If you are changing the Reference Rate Card of Projects in bulk
Update the Rate Card Name using the Projects CSV
From the Project Planner > Idenify the projects which need to apply the new rates
Click on the 3-dot icon > Select Edit Budget
Go to the Project Rates tab
Click on the grey arrow at the column header to apply all the rates of the selected reference Rate Card to the Project
Repeat the same process for each project
If you are customizing project rates in bulk
Download the Budget Per Role CSV to update the project rates per role.