Assuming that you have read the Start Menu skin tutorial: http://www.classicshell.net/tutorials/skintutorial.html
and made your skin already.
In newer versions of Classic Shell, a number of additional features have been introduced in the Start Menu for which the skin files saw some significant changes/additions. The skinning tutorial has also been vastly expanded post the release of Classic Shell 4.0. If you created your skin for the Start Menu based on an older version of a Classic Shell skin (and don't remember which version), then your skin will continue to work however these new features will not be available and in some cases (e.g. split buttons or highlighting), some skin elements may not display properly. Therefore it is highly recommended to all skin creators that they update their skins again using the newest Classic Shell version skin as the base skin.
The newest skins incorporate newer graphical assets required for split buttons (added in 3.6.0), highlighting new programs (added in 3.9.0), Aero colorization related changes (made in 3.6.0), proper single and two column style support (added for ALL skins now in 3.9.0), and numerous other changes (No icons in Second column, various bug fixes etc) made as late as 3.9.5 RC. Constant improvements are made post 4.0 release as well such as improved high DPI/high resolution support in skins, skin settings and tooltips getting localized, and support for masks for most skin bitmaps. In Classic Shell 4.2.1, some major improvements to the skinning engine were done again - specifically, the ability to customize the color of the search box and the scrollbar, using Metro colors, mixing up to 4 colors using the mask and combining colors for things like the text color or background color.Also, with version 4, a new format with .skin7 extension is introduced to support the brand new Windows 7 style of the menu with an inline treeview of 'Programs' and a number of other features such as the user picture jutting out. Existing Classic Shell skins are incompatible with this Windows 7 style, however all older skins will continue to work with the classic styles. If you want to create a skin for the new Windows 7 style, then you must base it on any skin with .skin7 extension and tell your users to switch to the correct style to use that skin. The older skins continue to carry the .skin extension so if you wish to simply bring your skin up-to-date, use a .skin file as the starting base (note .skin means classic styles of the menu).
Classic Shell 4.2.1 introduced new skinning abilities such as using Metro/Modern UI colors, skinnable scroll bars and search box etc. See this post
for more details.
Lastly, when releasing a skin, please state the version of Windows for which your Start Menu skin is intended and inform users to switch to the appropriate style. Classic Shell runs on Windows 7 and Windows 8 (and older versions supported Vista too). Skins look different depending on the OS.