You can migrate your EF4.5 to EF5.0 without the need of Microsoft .Net but if you use some features like the support of enumeration that provide EF5.0 you will need to have the .Net Framework 4.5.
What does it means? It means that you need to have Visual Studio 2012. So a simple migration of Entity Framework 5.0 became an endeavour of few hours but at the end work well.
I think it worth it if the business you are working on accept to change their tool (VS2010 to VS2012).