Lets be real for a minute, if you're in a bind and the baby changing cubicle is the only space available/accessible, then Id say go for it, but obviously don't try to sit your grown-up body on the changing platform. The thing is, at least in my country, baby changing, accessible toilets and changing places units (adult changing facilities) all seem to be rolled into one room usually, so there's a bit of an overlap of communities all trying to access one very limited space at all times. No one person has more or less right to access the space understandable, if you've had an accident, are uncomfortable, then you need to deal with it for both the comfort of yourself and technically the people around you, so yes, occupy whatever private space is available in a way that is respectful, leave it cleaner than you found it, etc.
Its pretty awkward, i really hope we can see an increase in these accessible facilities so that these aren't questions that need to be asked, especially considering the growth in people that require them.