I know we can configure worker log levels using --defaultWorkerLogLevel=level option through command line while running dataflow job directly from local or while creating template.
But I want to know how to use this option while invoking dataflow job from previously built templates where we didn't use this option while building it.
You should be able to simply set that parameter when invoking the job from a previously built template (either with CLI or API) - you don't have to recompile and stage the template again.
Format of parameters when using gcloud CLI is described well here Executing a Dataflow job with multiple inputs/outputs using gcloud cli
If you're using the API it depends on the SDK you have chosen - for example, Terraform makes it easy https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/dataflow_job#parameters
