I went the other way - 7539 to 7518
I got a 7539 plung for cheap, but I already had a Sidewinder lift and wanted to use the motor out of the 7539 for it. I had to convert my 7539 to a non-plunge 7518.
It would be expensive to do this, because the plunge base is around $100. IT was cheaper to convert the other way, as I bought the non-plunge base for $45.
So, if you want to do this, goto somewhere like eReplacementparts[d]com or another site, and look at the parts you need. The arbor is the same, as well as the motor parts. In addition to the base, you will need the springs, and the momentary-on switch too ($27), as well as the 2 plastic handles for the plunge base.
The most dofficult part for me was getting the bottom arbor nut removed, but I had air tools to make it easier. The rest of the router is fairly simple to take apart and put back together, much easier than it looks. PC seems to keep things simple with their designs, everything goes where you would logically think it would, and few if any parts leftover when you are done.