Like many developers, I like to get a pre-release look at software to get ahead of any major issues that I may face. Fortunately for me, our systems administrator likes to get an early look as well and sets up a separate instance for us to test the upgrade. 2020R1, at least in the pre-release build 20.093.0108, requires installing .Net Framework 4.8, both on the application server and on you development environment. Customization projects built in Visual Studio will need to be updated to .Net Framework 4.8 and recompiled in addition to any code changes you may need to make for “breaking changes” that come with the new release.
To install the .Net Framework 4.8 Developer Pack, you can download from Microsoft at https://dotnet.microsoft.com/download/dotnet-framework/net48. After installation, open your Visual Studio project and right click on each project. Select properties to open the Project Properties page and then select the Application tab. Select the newly installed .Net Framework 4.8. You will get a warning that a save and reopen will be performed if you continue, so just accept that and continue on. When reopened, recompile, and your code should be ready for 2020R1 once you resolve any compiler errors you may receive.
I learned this the hard way by struggling to compile my code copied over to the 2020R1 instance. This morning, I did some research via Google and came across a very old Stack Overflow post with the same errors that I was receiving. For more information on the error caused by mismatched .Net Framework versions, you can read more at: https://stackoverflow.com/questions/5567945/type-or-namespace-name-does-not-exist.