Group to Role mapper breaks - URL too long?

Trying to migrate 96 permission schemes to roles, the URL looks like:

https://jira.example.com/secure/admin/SchemeGroupToRoleTransformer!default.jspa?selectedSchemeType=PermissionScheme&selectedSchemeIds=10254&....

This failed with a 400 "Bad Request" error. I think this is because the total URL length is 5666 characters, and this is exceeding some limit.

The page should POST the form results to avoid this (and as it's modifying server state, it ought to anyway).