The following Preserved Architectural Remnants epitomize the category:
Archway from Third Young County Courthouse - Graham, TX
Note that Courthouse was razed when its replacement was built.
Italian Hall - Calumet, MI
Also razed.
Colonnade of North Wing of the New Market Block, Portland, Oregon
North wing was demolished.
Adams Hydroelectric Plant Entrance - Niagara Falls, NY
Original building Demolished.
Kenure House - Rush Co Dublin Ireland
Original building Demolished
1883 Stephens County Courthouse Arch - Breckenridge, TX
Original building Demolished
St. Boniface Cathedral - Winnipeg MB*
"newer smaller cathedral has been built inside the
remnants of the former cathedral."
Columbus Union Station Arch - Columbus, Ohio
Original building demolished.
Lineham Block - The Hyatt Hotel - Calgary, Alberta
New building constructed using the old facades.
St Leonard's Tower, Newton Abbot, Devon UK
Rest of the church demolished.
Arco do Presídio - Sao Paulo, Brazil
Original structure demolished.
Please note that this does not mean the others are WRONG. But the ones
above are the kind of things I had in mind when I created the category.
*I like this one so much I nominated it as a featured waymark and it was accepted.