Some additional searching on changes to code access security revealed a very helpful article on migrating partially-trusted assemblies. What I posted yesterday about preserving the previous behavior is found a little over halfway through the article, in the Level1 and Level2 section.
One thing this new article makes clear is that use of SecurityRuleset.Level1 should only be used as a temporary measure until code can be updated to support the new security model.
Code access security is one area that has changed quite significantly between .NET 3.5 and .NET 4.0. Once an assembly has been upgraded, if it allowed partially-trusted callers under .NET 3.5, it would throw exceptions when called under .NET 4.0. In order to make such assemblies continue to function after being upgraded, AssemblyInfo.cs needs to change from this:
[assembly: AllowPartiallyTrustedCallers]
to this:
[assembly: AllowPartiallyTrustedCallers]
[assembly: SecurityRules(SecurityRuleSet.Level1)]
Once this change has been made, the assembly will work under the same code access security rules that applied prior to .NET 4.0.