I found a workaround, although it isn't as clean as just re-linking. If you save the new file with the same name as the previous file, then replace the file with the new one, motion will automatically relink to it. If you add layers I'm not sure if it will recognize them or not. But if your just editing already existing layers this method works.
Adding layers messes up the layer order and will most likely ruin things. Motion should auto update any Photoshop changes made. If it doesn't just close out the program and re-open it and it should update the Photoshop image.
I'm working on a large project with many layered photoshop files right now and I have a few tips for updating them without problems.
If you make significant changes, add layers or resize the image you might have to re-import (remember to import from the menu rather than drag and drop or you might not get the layer selection screen!) and then re-link each object under Inspector>Properties>Media>To>Layer. This is thankfully not always necessary.
Add a bit of extra space around the edges of your image with the crop tool. If you have to change the image size later, the X/Y positions in Motion get shifted around and you'll have to reposition everything. If you give yourself a bit of working room you can just save in photoshop and it won't shift around on your canvas.
If you think you might need extra layers later, turn the visibility off on those layers in photoshop and import them into Motion. They will come in disabled but are available and positioned right if you need them later.
Plain raster layers are your friend. Start using a lot of smart objects, text layers, masks, layer styles etc. and Motion can get a bit unhappy and crash a lot.
Very large images can cause problems. I try to stay under 4000x4000px. Also make sure you are working with square pixels. Some of the photoshop "video" templates use non-square pixels and while they still import, the quality is noticeably lower. Of course, make sure you are working in RBG color and I've always used sRGB color profiles with good results.