First of all, we have to clarify, that word classes do not tend but are closed or open.
The closed classes are determiners, pronouns, prepositions and conjunctions. All these classes have in common that they are either function or structural words. They can be seen as concepts that structure the language which surrounds these words, but on their own they are meaningless.
If we just look at prepositions this becomes pretty obvious. In for example carries no real meaning. It is a word that shows the position of something else but without any further context it makes no sense. We can now add for example the derivational unbound suffix -side in order to create Inside. Now we might understand what the other person wants to say. Inside is not not anymore a normal preposition. It can be also used as a adverb or noun because it has a meaning now. When talking about the closed class of preposition we are talking about the core, the so called prototype, prepositions and those are clearly closed and not open. Some now might argue that despite and all these other forms may be "new" prepositions but if you break them down to the core they refer to one of the prototype prepositions. In the case of despite it would be "in spite of". In order to create a new preposition you have to create a word that shows a new connection between things and unless someone can do this they should be seen as a closed class.
Thus I don´t think that any of us ever recognized a change of open to closed or closed to open classes. When you look at the dimension of language (Halliday) it would also make no sense that they change.