Is there a reason the list isn't being updated to fix the data? If you have a reasonable number of rows/locations, you can filter the list for "garage 1", and then use the grid view to update all rows to "garage external 1", and then repeat for other locations. Doing that would be far preferable to modifying an app to deal with old data.
Otherwise, perhaps you could create a list in sharepoint with two columns, the old and new location values. When the user picks an item from the dropdown (in the onchange event), you could run something like:
Set(varOldLocation,LookUp(LocationList,Title=ddlLocationPicker.Selected.Value,OldLocation))
You could then modify the gallery filter to use an OR to specify two conditions, where the location should be either the selected location or the value of varOldLocation.
