Up to this point everything was easy. If you use old plugins, such as old galleries and guestbooks, it can get complicated.
Some plugins may not work, and you have to find alternatives.
Almost all old plugins are ANSI encoded. That means, you have to absolve the same procedure as for the content.htm of CMSimple, most of all for the language files of the plugins.
To mention all the hundreds of plugins for CMSimple, would go beyond the scope. If problems occur, simply register in the CMSimple Forum and ask for.