After Effects links to source footage so if you want to change a logo in a bunch of different projects that all refer to the same logo file just go to the original folder that contains the logo, rename the old logo, then create a new logo that has the same name as the original. If the logo has any animation applied to it or is repositioned in any of the comps the new one must be the same size as the original. IOW if your original logo is 300 X 600 pixels then the new one needs to be 300 X 600 pixels. One can be square and the other can be a circle, but the dimensions must be exactly the same when you examine the properties of the file.
Rick's instructions presume that you are using a pre-rendered video file of your logo, which is the most practical way to do the job, if you can.
But, to answer your question more specifically, you cannot create a dynamic link to one AE project from another AE project.
Thank you both!
Too bad I cannot create a dynamic link...
I went with pre-rendering...