[Sekt0r]

If I already have separate NFO's that I've exported using the standard built-in feature in the Kodi settings and I now do another full export choosing to overwrite the existing NFO's, nothing should actually change from the old NFO's to the new NFO's, is that correct?
Assuming I haven't made any manual changes in the UI or via add-ons etc (which would be included in the new NFO's if I had), and assuming I have already refreshed/rescanned any changes in to Kodi that I might have made to any of the old NFO's manually (via Notepad++ etc).

Apologies for the basic question, I think I've had an incorrect understanding in my mind for awhile and coming across the NFO export page in the Wiki where it states the NFO's are only used as a Backup once scanned made me realise I've probably been doing something unnecessarily. 
There's no potentional downsides to overwriting the NFO files that I haven't mentioned? All I want is for what I see right now in the Kodi UI relating to my video library content to be preserved both in the UI and in the NFO's (so I can refresh at any time and revert back to the current state) regardless of how that information came to be populated (whether by scraper, manually, parsing file etc).

[Karellen]

nothing should actually change from the old NFO's to the new NFO's, is that correct?

Most items won't change but it depends on your usage. There is the playcount and lastplayed values that might change if you watched it since the last export.
But if you have done none of that, then the existing nfo will be overwritten by a new nfo which is exactly the same.

the NFO export page in the Wiki where it states the NFO's are only used as a Backup once scanned made me realise I've probably been doing something unnecessarily. 

Yes, once the nfo files have been read to create the library, they are not used again, unless you do a Refresh, or rebuild your library. In light of that, then they can be thought of as a backup to your library.

There's no potentional downsides to overwriting the NFO files that I haven't mentioned?

Nope. I scrape my media the first time, then export to nfo files and then always use nfo files for backup. I run an export every few weeks to preserve any changes I have made.
If you have to rebuild your library, it will be exactly the same as it was when you ran the last export.

Also...
NFO files are only one half of the equation. The other half is artwork.
NFO files and Local Artwork go hand in hand. This will give you a proper backup.
Why?... Because the provider sites delete their artwork. So many of the artwork links in your nfo file may be dead, so the next time you rebuild the library, Kodi will try to find artwork using those dead links, and you will end up with missing artwork in your library.
Install Artwork Dump to download the artwork. Don't export artwork using the Export function of Kodi as it is modified and made smaller for faster loading.

[Sekt0r]

Great, thank you.
I hadn't thought about the playcount/lastplayed but I don't see that as big deal personally, if it's only since the last export (I use importwatchedstate/importresumepoint=true, which I read recently Kodi v20 will default to that; I think that change makes a lot of sense).

And I was aware of the Artwork part of the equation, I do prefer unaltered as possible when it comes to the display/storage of artwork and only export it using Artwork Dump (I already use imageres, fanartres and imagescalingalgorithm, also looking forward to Kodi v20 for imagequalityjpeg=0).

[scott967]

I like to have the playcount and resume point saved to nfo so I frequently do a video separate file without art export.  But I also will add tags to videos as well.  I think in Kodi 20 import of resume and playcount from nfo will be the default (currently you need to enable in advancedsettings).

scott s.
.

[themusj]

I had almost the same type of confusion.

I have exported my .nfo files routinely, but never overwrote them for two reasons.

1) I created some custom .nfo files for odd recordings, and wasn't sure what would happen to them.

2) My watched and resume states would disappear. I have my advancedsettings set to true.

If I now decided to overwrite them, what would be the results to 1 and 2 above. 

(I also use the add-on WatchedList as a backup).

[Karellen]

1) I created some custom .nfo files for odd recordings, and wasn't sure what would happen to them.

It will be overwritten by a new nfo that contains the same data. Or modified playcount data if you watched it since the last export.

2) My watched and resume states would disappear. I have my advancedsettings set to true.

Why do you think they would disappear?

[scott967]

Editing data nfos I think needs to be done with some care.  I like to first export with overwrite, so I know my nfos are consistent with the library, then make my edits, then refresh the affected library items.

scott s.
.

[themusj]

To be honest, a long time ago, for some reason, I thought that was the situation. Maybe I didn't have initially the configuration in advancesettings set to "True."

I found this post on the subject and it made me question my previous understanding.

It was the reason I relied on the add-on WatchedList.

Bottom line as I understand it now is: "overwriting .nfo files will save (and update) to the most recent watched state and play count of any videos viewed in Kodi."

Thank you for your great and timely response as normal.

[Sekt0r]

To be honest, a long time ago, for some reason, I thought that was the situation. Maybe I didn't have initially the configuration in advancesettings set to "True."

I found this post on the subject and it made me question my previous understanding.

It was the reason I relied on the add-on WatchedList.

Bottom line as I understand it now is: "overwriting .nfo files will save (and update) to the most recent watched state and play count of any videos viewed in Kodi."

Thank you for your great and timely response as normal.

Thought I'd just give you some quick reassurance that I performed an export+overwrite yesterday inpreparation for some batch changes I made outside of Kodi, followed by a completely new Video database/Library rescan. And all is well and as expected.

Just in case anyones wondering, the reason for my Library restart was because I had a lot of Movie Sets without Overviews (one of the batch changes I did) and from my understanding that particular attribute is only populated/refreshed in Kodi by the first movie added into a Set, which doing manually would mean removing all movies from Sets and re-adding.