What are the inherent risk of upgrading a website from the 2.0 framework to the 3.5 framework?
I know the features given to me in the upgrade, just curious if there are any known issues that may pop up when upgrading.
I.E. Function X used to behave this way, and now behaves a new way.
I have had very few problems whenever I have done this.
3.5 actually runs as 2.0 in IIS so really the 3.5 things are extras.
Give it a crack and compile, should be fine.
The only major 2.0->3.5 trouble areas I have encountered were in the Web.Config and centered around references to System.Web.Extensions 2.0. If you use the Visual Studio interface to change your project's .NET version, VS will automatically fix most reference incompatibilities in your Web.config file.
As for code differences, you're likely to find that some methods which have been obsoleted by newer versions, but I've never encountered any that outright break.
Microsoft holds a very strict backwards-compatibility standard so that they can reduce the impact of major upgrades as much as possible.
AFAIK there's only one issue you should check after targeting your project to .NET 3.5: web.config file changes. There's some info on this subject here on SO.
Other than that .NET was heavily extended between 2.0, 3.0 and 3.5 versions. But library core is pretty stable in what considers its functionality behavior.
One more note. I upgraded 3 different web projects without any problem and not even touching web.config.