We are using the java agent to monitor service calls etc. There are several features such as slow query logging that new relic agent offers, but we don't want to have switched on by default due to performance overhead etc.
If we want to enable them on prod by modifying something (ideally env vars, but worst case the newrelic.yml), will new relic pickup the changes after a period, or does it require a restart?
Most configuration options will only be read at startup.
From Config file docs.
Property changes to log_level and audit_mode do not require a restart. Property changes under circuit breaker don't require a restart.
Though there may be more. Check the specific configuration you want to change on that link to see if it mentions not requiring a restart.