diff --git a/lib/prometheus/client/data_stores/direct_file_store.rb b/lib/prometheus/client/data_stores/direct_file_store.rb
index b607f049..7ea6032a 100644
--- a/lib/prometheus/client/data_stores/direct_file_store.rb
+++ b/lib/prometheus/client/data_stores/direct_file_store.rb
@@ -68,6 +68,9 @@ def validate_metric_settings(metric_settings)
         end
 
         class MetricStore
+          # Update the filename prefix when the file format changes
+          FILENAME_PREFIX = "metricv2"
+
           attr_reader :metric_name, :store_settings
 
           def initialize(metric_name:, store_settings:, metric_settings:)
@@ -168,12 +171,12 @@ def internal_store
 
           # Filename for this metric's PStore (one per process)
           def filemap_filename
-            filename = "metric_#{ metric_name }___#{ process_id }.bin"
+            filename = "#{ FILENAME_PREFIX }_#{ metric_name }___#{ process_id }.bin"
             File.join(@store_settings[:dir], filename)
           end
 
           def stores_for_metric
-            Dir.glob(File.join(@store_settings[:dir], "metric_#{ metric_name }___*"))
+            Dir.glob(File.join(@store_settings[:dir], "#{ FILENAME_PREFIX }_#{ metric_name }___*"))
           end
 
           def process_id