In The Valedictorian, Jason, Ben, Jim-Bob, and John-Boy leave the home and prepare to be shipped overseas. At this point, Olivia has been gone for some time. Rose, Jeffrey and Serena are all living in the house.
The next episode, A Decade of The Waltons, Jason, Ben, Jim-Bob and Olivia back home. Rose, Jeffrey and Serena are not mentioned (although, this was a common enough occurrence in Season 8)... it doesn't really make sense for this to be the next "chapter" in the story of their lives (as Earl Hamner himself describes the way that he sees each episode in A Decade of The Waltons itself). Due to the addition of the documentary-style moments, you could say that it doesn't fit in the timeline and it's not a real episode - but that's no fun! The actors are all in character for a good portion of the episode and I think we can fit it in.
Instead of it coming between Season 8 and 9, I think it can actually fit into Season 8 itself, specifically after The Home Front and before The Kinfolk - that way we havean explanation for John-Boy, Rose, Jeffrey and Serena's absence, an explanation for why Olivia, Jason, Jim-Bob and Ben all live there and it makes sense that Esther is not currently living with the family either. I think I may even watch it in that position in future, as it does seem odd to go from the boys marching off to war... to them immediately being back.
Where do you think it fits in?
Also, as a side note, it's a bit of a shame that Cindy, Rose, Jeffrey and Serena are all left out of the episode. They're a part of the "decade of The Waltons" and it feels especially odd not to mention Ben's marriage when they go over all the other "big" events in their life. Still a nice retrospective nonetheless
I’m guessing it was filmed at the beginning of season 8, before Michael left the show, before Rose and the kids showed up, and before Johnboy was recast. And then it was held for later. It’s an odd episode.