This is prolly the most stupidest Q in here but nonetheless. How do you replace an image with footage while saving all the keypoints. I know it's a simple shortcut but i forgot. 
Select you're footage that you're going to replace in the timeline of the compisition.
Then select the replacing footage from the project window.
Hold down Alt while you have the footage in the project window selected, drag it to the composition window..
bam, replaced...
Another way to do it if you wanted to globaly replace the footage, as in everywhere and have the other gone is to merly right click on the footage in the project window and select "repace footage file" or whatever and browse to the file you are replacing it with and bam, its all replaced globaly.
Hope it helps, I do it without thinking at this point so I hope I wrote it right haha.