Slides: areas in red are the focus. Newer P89 on left, P85/older P89 on right. Notice there's a defined channel that the ejector fits through on the older model (immediately left of where the firing pin fits in the slide), while the area is more open on the newer models. These slides are both safety/decock models, but the significant area (the channel) is the same in both the newer and older DC models. The DC models have a leaf spring and retaining pin in the open area to the left of the aforementioned channel, but this is not relevant to the fit.
Ejectors: On the left is the old style ejector, on the right is the newer style. Quite different shapes.
Make sure the slide you want to swap onto your gun has the appropriate shaped channel for the style of ejector in your gun. When you put the old style slide on a frame with the newer ejector, it can bind on the ejector, particularly with the magazine in place. As I said before, I'm not sure if the newer slide will work with an older ejector, as I haven't tried it.
It also appears to me that you could perhaps swap out the ejector in your gun to match the channel in your replacement slide, should they be different. Example: let's say your gun has the new style ejector but you find a great deal on an older DC slide. I looks like swapping the ejector currently in your gun with an old style one might allow the slide to work properly. I have not tried this either, so I'm not 100% sure.
I might try a few different combos of slides and ejectors this weekend and find out if an ejector swap is doable. Until then, I hope this helps.
I sort of goofed up the first pic. For additional clarity, ejector channels are marked in green:
Update: I put the old ejector into my frame and it allowed the old style slide to function just fine; however, I realized I could not completely seat a magazine in the gun, so I pulled the ejector back out and very slightly bent the little hook at the top out, so it would clear the top of the magazine. This cleared up the issue, but fiddling and fitting are necessary if you go this route. I'm really glad you asked this question, as I'm learning new things myself as a result.