I have a project of migrating a solution on top of MOSS 2007 to SharePoint 2010. The project is an intranet portal with 12 applications on top of MOSS 2007. The applications are 3 tiers (DLL,BLL, UI) the UI layer contains a set of web parts controlled by the BLL which in turn accesses the database through the DLL.
1. Upgrade you solution to be on top of MOSS 2007 SP2 and WSS 3.2 SP2.
2. Compile the source code for the solution using VS 2010 and choose the .Net framework 3.5 as your target framework.
3. Replace references to all Microsoft.SharePoint assemblies with the new SharePoint assemblies you may use the below this article to solve the bugs resulted from changing references
4. You can find a list of assemblies and name spaces changes between MOSS 2007 and SharePoint 2010 in the below link
5. You may need to add reference to SharePoint assemblies from "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\ISAPI" such as "microsoft.sharepoint.portal.dll", if you cannot find them in global assembly cache.
I will complete this article in part 2 soon , once I have time to write it.