How to make backward compatibility of windows phone 8.1 app for windows phone 7?

Question

I developed windows phone 8.1 app which is in windows phone store. Now, i want to make this app compatible for windows 7 also. Is it possible, if yes then how?

Answer 1

It may be that you are using non-backward compatible capabilities in your Windows Phone 8.1 app(s).
Developers that want to develop for different OS's usually have platforms with the different OS's on them for developing applications so they work on each platform.
I suppose you developed for Windows phone 8.1 rather than Windows phone 7. Therefore your app runs on a different framework than a Windows phone 7 has as well as probably newer hardware with greater capabilities.
So you will probably need to purchase an older phone that came with Windows phone 7 in order to develop for. I couldn't find documentation like the last link below (App capabilities and hardware requirements for Windows Phone 8) for Windows phone 7
from Microsoft but maybe you can.
Windows Phone OS 7.1 - Developing a Windows Phone Application from Start to Finish
Types of Applications
The Windows Phone application platform provides two frameworks for developing applications:
Silverlight
The Silverlight framework supports event-driven, XAML-based application development.
XNA
The XNA Framework supports loop-based games.
Silverlight and XNA can also be combined in a single application.
The following table lists some of the criteria that you can use to determine whether you should use Silverlight or the XNA Framework for your Windows Phone application. With Windows Phone OS 7.1, you can combine Silverlight and XNA into one application. 
For more information about Silverlight and XNA, see
The Silverlight and XNA Frameworks for Windows Phone. For more information about the application platform, see
Application Platform Overview for Windows Phone.
What's New in Windows Phone 8.1
Platform convergence with Windows Store apps
Windows Phone 8.1 introduces an important change in the Windows Phone developer ecosystem. In this release, Windows Phone converges with the Windows Store apps platform into a single developer platform that runs the same types of apps—Windows Runtime apps.
Platform convergence began in Windows Phone 8, which supports a small subset of Windows Runtime APIs, but which differs from Windows in many core areas. In Windows Phone 8.1, there’s so much more in common—a much larger API set, a similar app model
and life cycle, a shared toolset, a common UI framework—Windows Phone and Windows Store app developer platforms truly have become one, single development platform.
Of course there are still some small differences in behavior and supported features between Windows Phone and Windows Store apps. Some of these are the result of timing of the different product cycles, and they may not appear in future releases. Some differences
are the result of the different natures of phones and computers, their sizes, and the way people use them. We encourage developers to think in terms of Windows app development—developing for a single Windows Runtime, but targeting two different platforms on
phones and computers.
Required Tools for Windows Phone Development
App capabilities and hardware requirements for Windows Phone
8
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