Marx sees change as being driven by the struggle between classes. From this it follows that in order for there to be change (specifically towards equality), there has to be a struggle. And in order for a struggle to exist, inequality must exist. Although it may at first seem paradoxical that inequality drives change towards equality, it makes sense for a couple of reasons. First of all, if there was no inequality, no change would be needed. Essentially Marx is saying that change is driven precisely by the need for change. It also makes sense that worsening  inequality can actually lead to faster change because it makes the oppressed class feel the need for change more acutely. Therefore they are more likely to take action, which is necessary to make progress in the struggle against the bourgeoisie. An example of inequality intensifying the struggle between two groups in the modern day can be seen in the way  police shootings have led to the Black Lives Matter Movement and to race relations getting more attention in the public eye. Hopefully, these things will eventually lead to greater equality for African Americans, which would be consistent with the Marxist model.