I am wondering if re-using a roller tray liner is asking for trouble. The old paint was white primer; the new paint is very slightly different white. Even if I scrape out all the possible flakes and thicker chunks of old paint, I'm wondering if the new paint will loosen up the old and create a mix / mess? I'm using Benjamin Moore Natura (White Dove OC17) on top of BM Natura zero VOC interior primer White 511 00.
There is a good chance that you will be perfectly fine. I've rolled out of trays I've let paint dry in with perfectly fine results when I got lazy and it was an area I wasn't concerned about (like a shed, inside of my garage, etc...)
However, based on which paint you chose, I'd say this is more important area, so why risk it?
There is a chance that the new wet paint plus the bending, flexing, or pressure of you rolling could loosen up some more pieces/flakes.
Liners are cheap, but if you really do plan on reusing them clean them up immediately after use to make it easier.
I wouldn't re-use one of those flimsy disposable liners if the paint already dried on it. When the liner bends, the old paint will chip off and end up stuck to your walls. The best solution is to get a roller screen that mounts inside a 5 gallon bucket ($5 for the screen, and you probably have buckets somewhere). That way you can dip your roller, roll it on the screen a couple times quickly, and any splatters stay inside the bucket. This method is MUCH faster and less messy than a roller tray. When you're done, just rinse the bucket and screen out with water if you're using latex paint. If a little paint dries on the bucket or screen, it won't matter since they aren't flexible enough to make the paint chip off. I have roller screens that are about 3 times their original thickness from years of paint layers.