Solid puzzle. Loved the entry at 11-D11DThe DD in DDR[----------], with a fun clue too. The theme is really clever and a good moment of victory when discovered with the cyclic substitution you used, but I'm not sure the revealer works. Something should indicate what's happening to the first words.
Sore loser's retort, as in a story by 49-down49DFabled fabler[-----]
38A
Wes Anderson's "Isle of ____"
39A
Protagonist of "Solo"
40A
"Cake by the Ocean" band
41A
Select a choice few
44A
Easy, in Ecuador
45A
Common elementary sch. course for immigrants
46A
With mi, a Vietnamese sandwich
47A
"It's a ____ __ chance"
48A
Common song refrain for Britney and Bieber
50A
Clumsy one
51A
Skillful
53A
Produces, or a hint for 20-across20AMarvelous[----------], 36-across36ASore loser's retort, as in a story by 49-down[---------], and 41-across41ASelect a choice few[---------]
60A
Southeast Asian country with capital at Vientiane
61A
Admiral Ackbar: "It's _ ____"
62A
Approximation words
63A
Potato in South Asia
64A
Milk container
65A
Bert and Ernie or Key and Peele
66A
Signal disruptor, for short
67A
Vandalized a car
68A
If counterpart
Down
1D
Catchall cat.
2D
Disapproving sounds
3D
Savory Oaxacan sauce
4D
Pointy boot accessory
5D
Listened to Sam Smith or remained
6D
Oft-separated egg components
7D
Leg joint
8D
Roof overhang
9D
Slim
10D
Biodiverse Indian state home to many Indian rhinos
11D
The DD in DDR
12D
Petri dish filling
16D
Orangutan or gorilla
21D
Loaves for Reubens
25D
Dadaist artist Jean
26D
Blue _____ Mountains
27D
Smells
28D
Rapid growth of cyanobacteria caused by runoff
29D
Pre-matriculation summer program for STEM students