I'm not sure what tab you're talking about, unless you were using the Coordinate System tab to change the data frame's coordinate system. That would change the reported extents for the layers/data frame because the data is now being projected on the fly in ArcMap.
Extents usually reflect the actual coordinate values in the data.
There are several ways to permanently change the extents of a dataset.
1. Create a new dataset by projecting the data to a new coordinate system.
2. Use the spatial adjustment or georeferencing toolbar or something similar to 'move' the data to a new coordinate system.
ESRI Product Specialist