Steve,
You shouldn't need to upgrade your module to work on DNN 8 because DNN 8 is still based on webforms . You may already know that.
Your existing project will also work if moved to a DNN 8 environment assuming when you originally built it, all of the DNN references in the project were relative to the /bin folder. The only change you would need to make in order to compile the module with DNN 8 assembly references is to change the .NET version in the project properties to .NET 4.5.1.
Regarding templates, the older DNN 7 templates still work for DNN 8. I see no reason for you to use a DNN 8 template if you are sticking with the webforms module controls. You can still use the "DNN7 DAL2 C#" template and, like I mentioned ealier, bump up the .NET from 4.0 to 4.5.1 so it compiles in a DNN 8 environment. The new DNN 8 templates can be installed in addition to the DNN 7 templates because they only have the new type project structures for MVC and SPA as you know.
If you really want to move the module's codebase to one of the new DNN 8 types, decide which one is best for your module. Check out a blog I did on this topic. If most of your module's logic is in javascript/WebAPI, the easiest conversion is SPA. You can pretty much rename the module control files from .ascx to .html (in your manifest too) . Obviously you would also translate the server tags to the SPA tokens in the .ascx content too.
If you want to move to MVC, you need to change a lot more. I think either way, I would branch my existing code and start a new project from scratch and migrate the code from the old project to the new one. But leave the old one in tact for backward compatibility.