As i stated in my previous response, the article i posted there does exactly what you are asking for. The only reason he creates a new project is to copy the content that you will have to change in your app.json and launch.json. So there normally is no need to create a new project.
This is the way to go in my opinion.
Now that being said even if you create new project (which i don't advice unless you can't otherwise) you wouldn't have to rewrite your entire code. You would only have to move the files from the project (pages, pageextensions, tables, tableextensions, codeunits, ...). These files are normally in the "src" folder.
So my advice on how to do this is:
- Create a new project for BC15.
- Set up your app.json and launch.json
- Download symbols
- Copy the content of the "src" folder in BC14 to the "src" folder of your BC15 (COPY don't move you never know if something would get corrupted)
- Generate your translation files
- Build your project
Regards.
