Help+Manual automatically converts the project files from older versions with obsolete formats (H+M 3 and H+M 4) when you open them. Alternatively, you can also use the standalone converter program to convert old projects manually.
Help+Manual 4 projects are generally converted 1:1 and do not require any post-processing after conversion. Older Help+Manual 3 projects did not have the styles and other features introduced in the modern versions of the program and may require some reformatting after importing.
The Help+Manual 5 and 6 project formats are almost identical with the current format and require no conversion.
You can edit projects saved with Help+Manual 9 in versions 5 and 6 but this is not recommended. Any new features will be stripped out by the older versions when you do this. If you are working on projects in a team all team members should also be using exactly the same version.