Tvheadend

Hi,
I have a customized IPTV List that comes from various sources. Some of them get scanned with the default HTTP User-Agent, while others need an empty or something like VLC or ffmpeg. If I scan the M3U and then manually select the failed muxes and change the User Agent, they will be found; however, every time I force the scan of the M3U (maybe because I changed something - I was playing with the XML Guide and changed some EXTINF data), I need to manually re-input the customzied User-Agent field.
So, two questions:

- If I change the EXTINF (like the tvg-id field, for instance), should I in any case force the scan?
- How can I preserve the user agent?

Thanks!

Using HTS Tvheadend 4.3-1979 ~ LibreELEC Tvh-addon v10.0.0.102