An album can remain exactly the recording you collected while a compatible player changes its playback gain. That is the useful idea behind ReplayGain tags: a playback instruction is different from producing a newly mastered audio file.
Check which operation you are choosing
A scan, a tag update and a command that alters audio are not interchangeable. Read the player’s wording before accepting it. The foobar2000 ecosystem even documents an Alter File Content operation, so seeing ReplayGain in a menu is not enough to establish that the selected action is metadata-only.
Work on copies for the first experiment. Keep the originals and note the player version, selected action and playback setting. Then turn ReplayGain processing on and off while listening at a comfortable level. A changed listening level does not mean that detail has been restored to the recording.
Keep the album relationship in view
Track-based and album-based playback serve different listening situations. A shuffled collection and an album with deliberately quiet passages do not pose identical questions. Compare a quiet track followed by a louder one, then listen to the transition within the complete album.
Record which mode you used rather than writing only “normalised.” If the same tagged files sound unchanged on another device, check whether that player reads and applies the tags. Metadata can travel with a file without every recipient acting on it; a useful collection note explains both the file and the playback path.
AI-generated editorial illustration; not a documentary photograph.