>Hi everybody,
>
>In the big solution we'd like to have one project to target .NET 3.5 while the rest of the projects are targeting 4.5 .NET. Do you know what implications this may have?
>
>Thanks in advance.

See http://stackoverflow.com/questions/4359266/net-3-5-application-on-net-4-0-runtime for various resources indicating what the potential issues might be. The article at https://msdn.microsoft.com/en-us/library/dn458358%28v=vs.110%29.aspx has detailed migration issues for moving to 4.5.