We will be installing .NET Framework 4.5 on our servers in next few days starting from 10/03 - 10/06. .NET framework 4.5 Installation is an In-Place upgrade of 4.0 to 4.5 and NOT a side-by-side installation, it actually replaces entire .NET Framework 4.0 all its assemblies with 4.5 .NET Framework version DLLs.
The websites on our servers that are currently targeted to run on 4.0 should just work fine on 4.5 Framework. To migrate your websites to .NET 4.5 Framework all you would do is - open up your projects in Visual studio and target its platform to .NET 4.5 Framework.
If you need any further information or support, you could contact us through tickets or Microsoft .Net forums. We will be to help you working with ASP.NET 4.5 Team of Microsoft to resolve any of issues if faced w.r.t 4.5 upgrade. Please do refer to below links for more information about .NET Framework version 4.5 installation and migration.