Microsoft released Windows Phone 8 last October which includes lots of new features in the Windows Phone. Developers tend to have a doubt about their Windows Phone 7.5 apps. What will happen to their Windows Phone 7.5 apps?

Will the Windows Phone 7.5 apps work fine on Windows Phone 8 as well?

Yes, the Windows Phone 7 and Windows Phone 7.5 apps will work fine on Windows Phone 8 but it is to be noted that the Apps targeted or developed for Windows Phone 8 might not run on previous versions of Windows Phone. It is also suggested that the developers should test their Windows Phone 7.5 app on Windows Phone 8 to make sure that the app works fine on the WP8 device.

Check for the What is Quirks mode in Windows Phone? to know more about it