Since it's come up, I may as well ask about a whole cluster of tropes:
What makes them different from each other?
What I'm seeing so far is:
Please tell me where I'm wrong.
I've had my own doubts about Release Date Change being distinct enought to qualify as a separate trope, but...
As I've long understood the term -- which I first heard years before TVT was founded -- Development Hell is (or at least was) the pre-production and even pre-pre-production stage of film-making, where the creative team (or the studio) is attempting to whip a project into shape and promote it to management so that it will be greenlighted and become a genuine work-in-progress. EDIT: It's when this period extends past a reasonable amount -- more than a year or so, or when the project is abandoned -- that the project is said to have entered Development Hell.
Schedule Slip is for a work that appears in installments, ostensibly on a regular schedule. The work already exists -- it's just that the creator misses an installment, and the interval between the installments slowly grows.
Vaporware is "we've got this great program, it'll be coming out real soon now" but the program doesn't actually exist and may never exist. There is (or was, at least originally) an element of deliberate deception in vaporware, as it was usually employed to take the wind out of the sails of a competitor with an innovative new product by claiming your version of the same kind of product was just weeks or months away from release, thereby convincing customers to wait for your version rather than give money to your competitor right now.
Release Date Change is simply "the date it was to come out changed". Which could be any and all of these.
For how I see it, Development Hell is actually more related with Troubled Production, in the sense that both are about works whose production gets uphill. The difference is that the work embroiled in a Troubled Production eventually gets made at least partially (even if it is not released), with an intellectual property caught in Development Hell may not even get to the production stage, eternally promised but never leaving the planning or the pre-production stage. Vaporware is a particular variant of Development Hell specialized in video games and software, as with them is more easy to get proof that some work is being done (just release an alpha demo or build screenshots or something), but the overall effect is the same.
Release Date Change is the decision to either delay or push back the release date of a work. The reasons for it can be because the work got caught in a troubled production or was given a tentative release date in an attempt to get them from the Development Hell that obviously didn't take, but just as often it is applied to works that were finished on time but for whom their creators think they cannot be to release it on its initial given date for some reason (see The Shelf of Movie Languishment for this trope being taken to extremes). Schedule Slip is a subtrope of both Release Date Change and Development Hell, when further installments of a serial work are delayed for reasons that more often than not amount to "it's difficult to do this regularly and consistently".