[Link for reference: Reverse cards aren´t working]
If you used Change Note Type – it applies to all notes you have selected. It warns you about what will be deleted, but it’s up to you to consider how many notes you’ve selected.
If you did it by modifying the note type in the template editor, like by deleting a card type – the window tells you explicitly that it will affect all notes that use that type –
To get back what you lost (since it sounds like it’s too late to Undo or force a one-way sync?) – you can retrieve that review history from an automatic backup. If you’re not worried about losing anything else that has happened since then (edits, updates, other review history), you can simply restore the backup from just before you made the destructive change. If you’re not sure when it was, you can keep trying them one-by-one until you find it.
But if you have things you want to preserve that have happened since then, you will need to be more careful. It also matters exactly what those things are – e.g., newly added notes would be pretty easy, but review history on these same notes you’re restoring might be harder.