regarding file id change of a file.

If a file is deleted and then uploaded again, it is treated as a new file. That means it is no longer the same item as the original file, so the file ID can change.

If a file is updated by replacing it with a new file using the same name in a document library, SharePoint can keep it as the same document item when the file is checked out, replaced, and then checked back in. If versioning is enabled, SharePoint creates a new version and keeps earlier versions. That indicates an update/version operation does not create a new file item in the same way as delete-and-upload.

A practical distinction is:

1. Delete + upload again: creates a new file item, so a different file ID is expected.
2. Update/replace existing file: typically updates the existing document and creates a new version if versioning is enabled, rather than creating a brand-new item.

This matches the observed behavior where a renamed and re-uploaded file is treated as a new file rather than the original one.

References:

• Edit uploaded files after a SharePoint transition
• Check out or check in files in a document library
• Only one PDF file does not appear in the File and Media web part on SharePoint. - Microsoft Q&A
• Is there a defined format for OneDrive (for Business) FileID? - Microsoft Q&A