Using LRT 5.8 and latest version LRc for workflow... I'd like to edit and export 1 or select number of frames to replace what was exported from LRc already instead of exporting the entire 900+ frames just because I wanted to change 1 frame. Is this possible without messing up the sequence when rendering in LRT? When I select 1 frame and export using the LRT plug-in, I get a warning telling me I need to select the entire sequence. There's no way to export and replace just 1 frame?

Yes, it's possible.
You need to select at least 2 adjacent images (1 wouldn't export as you already noticed).
Then under "Advanced Settings" check "Overwrite" and enter the image number of the first image to export under "Initial sequence number".
So for example, if you want to reexport image #234 you'd select image 234 and 235, then under initial sequence number enter "234". This will reexport only those two images overwriting the existent ones.

Is this still possible under LRT 6.5.1 ?

I have selected a block of two images, but i just cannot find the Advanced Settings Menu item

Yes, of course it is. But nite that we are talking about the export from lightroom via the LRTExport plug-in here.

Ah IK.
I only have an old Lightroom that the pluggin does not work with.
I’ll just continue to do complete export each time and let it run overnight.