While migrating SharePoint application from 2007 to 2010, we need to consider or look at or check the following points:
1) Absolute Classes and Namespaces - Because the changes in the API in the upgrades are backward compatible, we don't have to make any changes to 2007 custom solutions before we redeploy them to the 2010 version of the SharePoint.
Some classes and namespaces are absolute but they will continue to work as expected.We need to recompile our code. The compiler warning will tell us which elements of the object model are absolute and which newer alternatives we should use.
2) Event Handler - We need to recompile and redeploy it.
3) Master Page and Themes - SharePoint 2007 themes no longer exist in SharePoint 2010 so any customization and design work that we have done with themes will not be imported into the new version.
4) Scripts - Scripts should work with 2010, but we should check to determine whether they are relevant to the new changes.
5) 32 bit DLLs - DLLs that support 32 bit application will no longer be supported in SharePoint 2010 as it is designed to work in 64 bit.