Eight pairs of adjacent answers have sections - entire words that appear within the original words - that swap places. The revealer GOESUPANDDOWN (65A: What each group of shaded words in this puzzle does) gives another layer of complexity, in that each inner word that has been shifted up or down in the grid can be followed by the appropriate word, "up" or "down," to make a common phrase. END up, LIE down. Not bad. I don't particularly like that the Acrosses (as read normally) are rendered nonsensical, but I suppose I can live with it. It's a little odd, don't you think, but at least it's novel, and sometimes that's enough.
Every Across over seven letters is involved in the theme, but the Downs are still full of longish, decent fill. TERRACE (10D: Balustrade location), PITFALL (91D: Unseen danger), DUPLEXES (67D: Divided houses), and TAPENADE (8D: Appetizer with puréed olives) are all good. Mmmm…. tapenade…. SWAINS (64D: Courters), though it gets a slight deduction for being plural, is a great word.
Just last night we were watching "High School Quiz Show" on WGBH, and one of the questions was "What country has the longest coastline?" I was saying something just then and couldn't hear the correct answer, so I looked it up on Google, and I was sure glad I did when I saw 41A: Country with the longest coastline (CANADA), because if I hadn't, I wouldn't have allowed myself to do it during the actual solve. Phew!
So, to sum up, an odd theme, but well done. Thumbs up.