Well, it seems both sets were used during the show's run. There were a few more cosmetic changes, such as:
1966 to 196(9)
- Trees decor behind players
- Empty arch grids behind-top of players
- NG intro logo in a box
- Eubanks wearing suit with NG badge
- White/black text logo behind Eubanks
- Grid sides on chappel lectern
- Pink curtains w/thick tree branch patterns
- No big win music
196(9) to later
- No trees decor behind players
- Partially filled grids behind-top of players
- NG intro logo attached to horizontal wood stilts
- Eubanks wearing ordinary suits
- Blue/white text logo behind Eubanks
- Blue sided chappel lectern
- Yellow curtains w/thinner tree branch patterns
- Added big win music
There's no YouTube episodes of NG b/w the years 1966-1969, so guessing when they changed it I can't say. The picture from GettyImages shows different couples from the one on Youtube, and since a pilot wasn't made, there must have been lots more episodes made with that grid-y set.
As to why? Beats me, maybe it made the set less white, plain, & gridded.