Well, the set has gone though MANY changes.
1) The most DRAMATIC change happened between 1969 and 1970. Nowadays, the street curves around at a 90 degree angle, with a playground/tire-swing between Hooper's and 123 Sesame Street. Back in Season 1, there was none of that. Just a long street, with a fence connecting the store and the apt. building.
Reason why they changed it? Carroll Spinney explained in his book that the "L" configuration of the set allowed for more different camera angles, rather than looking waaay down the street either from one side or another.
2) Also, Oscar's trash-can set up has changed over the years.
- first half of 1969 - he had a blue box next to his trash can, sitting on top of a larger grey box
- second half of 1969 -1973 or 1974 the blue box changed to a purple trunk
- 1974 - present - Brand new trash can sits next to brown crate, on top of barrel
- 1978 or 1979 - Oscar's trash can is accidentally taken away by the sanitary dept, so he has Luis fix up (i.e., dirty up and bang dents into) a whole new trash can
3) The space beyond Big Bird's nest and the lamppost also has changed BIG time:
- yes, the cityscape that was painted on the wall has changed from 1969 to 1992
- and then 1992 or 1993, the set goes "around the corner" and reveals a whole new cul-de-sac, including the Furry Arms Hotel, Ruthie's Finders Keepers, and Celina's Dance Studio
- then in 1998, probably due to budget cuts or story limitations, that whole new corner magically disappears, and what appears there now is just a couple of colorful doors and a brick wall
4) The Sesame Street sign has changed too. Once CTW changed its name to Sesame Workshop in the late 1990's, they updated the Sesame sign.
5) Also: the set location has changed too.
From 1969 - 1992, the show was taped in an old movie theater on Manhattan's Upper West Side, 81st Street Teletape.
Now, you have to take a cab across the East River to Kaufman Austoria Studios in Queens, where the Cosby Show and Spin City were taped.
6) And over the years, they've "recreated" the set for various productions outside of the regular PBS show.
- For "JULIE on SESAME STREET", a 1973 ABC special w/ Julie Andrews, they made a strange recreation at ATP Studios in London - they actually revealed what was across the street from 123 Sesame
- And like you said, they had different interpretations of the studio set in FOLLOW THAT BIRD and ELMO IN GROUCHLAND. I know FOLLOW was filmed in TORONTO on a backlot.
Hope this helps!