Ok, you have 2 options: Individual line editing (Easy, slow) or Batch Editing (Slightly more complex, fast). You need to know how to do it individually to understand batch editing though.
Individual Line Editing
Requirements: OverlayEditor, Text Editor (Notepad++ recommended).
Locate a vanishing object (Overlay Editor is easiest for this!). In this case I know "AICM-GROUND-mex_grd01" is one of these in a raw MMMX conversion.
Locate the object in explorer, and open it as a text file (I use notepad++). Scroll to the bottom of the file.
There will be a line named "ATTR_LOD_draped" present. Change the value there to the distance (in feet) you want the item to be visible for. Lower values may give marginally better performance, ideally for fine details. Since this is an airport base image, I want to see it from further away, so I'll change the value to 50000, roughly 10nmi.
Load up the sim, and move the camera away. You should see that your object stays visible! Repeat for EVERY VANISHING OBJECT.
Batch Editing
Requirements: Batch Text Editor (I use Textmage for Windows, though principles should be similar for other products).
Select all objects in your scenery's objects folder.
Drag the objects into a new, open Textmage window to import them.
locate and select the "Edit lines>Replace" option.
Choose to replace "...lines by use of filters".
Enter "ATTR_LOD_draped" into the "Line Contains <text>" box. (see image in point 6)
Enter "ATTR_LOD_draped<tab>XXXXX" into the "Replace with" box (XXXXX is, as mentioned above, the distance the object will be visible for, in feet).
Click "save..." in the top-right corner of the textmage window. Uncheck "Backup as a ZIP archive" (It slows the process a lot if you have many files). Press Start and wait for the process to complete.
Open X-Plane and visit your scenery. Now every draped object (ground texture) will be visible using the LOD you set!
(Optional) If you aren't happy with the value you set, you can just change the value in the "Replace with" box if you kept textmage open, or repeat this process again and change the same value. Easy!
Once you're done, close textmage. You can ignore any further save prompts it displays.
The great thing about this process is you only have to do it once for an entire scenery - and it is much, much faster!
I hope this guide has been incredibly comprehensive. You can use it for any future conversions too!