I've had some wonkiness with the Network Sonar Discovery especially trying to get existing nodes to switch polling methods.
Fortunately, we can mass assign CPU & Memory resource under Settings > Manage Pollers.
If you group the pollers by Author, the CPU & Memory poller is under "SolarWinds Native Poller". Select that poller and click Assign. You can then apply that resource to all nodes that match the scan results.