Use Group Managed Service Account for SSRS

We have a production environment running SQL Reporting Services 2014. The service is running as a group managed service account domain\SSRSProdSvc$ We are now trying to change our test environment to use managed service accounts for SSRS, however I can't change it. If I go into Reporting Services Configuration Manager and try and change the service account I get an error > Setting Windows Service Identity to Windows Account > > The Windows account information used to run the Windows service has failed to update. Previously designated settings are being used instead. If I go into Windows Services I can change the account to domain\SSRSTestSvc$ and the service starts, however in the Reporting Services Configuration Manager it still comes up with the previous domain account. I'm not sure how it was configured on Live, but when I look at the Reporting Services Configuration Manager it's service account is domain\SSRSProdSvc$ Any ideas how to fix test?