Going through the Wiki, I've figured out the timeline for the gradual overhauling of the Arbor area during the late 80s/early 90s:
- Season 19: The backdrop behind the fence is completely replaced, and the brick alphabet facade is a textured structure at the beginning of the season. Also, the brick siding on Hooper's Store as well as 123 are also gradually built up with actual brick texture throughout the whole season. The trellis outside Hooper's side door is also downsized partway through the season.
- Season 20: The facade of the carriage house is replaced with actual brick texture (also, I think this is the season where Big Bird's next suddenly has partially torn-down brick walls and boulders around it).
- Season 21: The backdrop behind the fence fills in the empty patch of sky with the additional distant building, and the alphabet facade is fitted with a fire escape, and closed-off factory windows.