I created app for Framework 2.0, but suddenly get know, that i need to make it on Framework 1.1. I tried to rebuild my project on required version Framework as i did it before(from 4.0 to 2.0), but unfortunatelly visual studio 2015 update 1 haven't got such version at the list to choose. Is it ever possible to rebuild it somehow? I googled it, but didn't find anything.
Visual Studio 2003 is the last version to support .NET 1.1. So you'll need to port it back to that version. Depending on what you used in the framework, there can be a lot of breaking changes when backporting. Unfortunately, the release notes only describe the breaking changes when going from 1.1 to 2.0, not the other way around.
But I highly recommend not to do so. .NET 1.1 is old and it's no longer supported, neither is Visual Studio 2003:
.NET Framework 1.1 SP1: The .NET Framework 1.0 was supported at the latest Service Pack level (SP1) under an individual product lifecycle policy until October 8, 2013. However, .NET Framework 1.1 SP1 will continue to be supported until end of support of Windows Server 2003 SP2 (with 32-bit only, not 64-bit).
Note that the extension does not apply to Windows Server 2003 SP2 64-bit edition and Windows Server 2003 Itanium based systems where .NET Framework 1.1 did not ship as a component in the operating system.
https://support.microsoft.com/en-us/gp/framework_faq/en-us
Microsoft Visual Studio .NET 2003 Service Pack 1
Lifecycle Start Date Mainstream Support End Date
9/15/2006 10/14/2008 10/8/2013