Hi Olle,
I'm into this at the moment as well.
For a long time I used to do this much like you do with an mpl playlist but this required a lot of external editing : 1)create an mpl playlist with your existing files 2)edit the mpl playlist with an external editor (I use the excellent pspad) and change the old files with the new ones 3)import the changed mpl playlist 4)updates the tags and library (library tools).
With the recent MC12 release (12.0.470), new opportunities to do this have emerged. You can escape the mpl playlist method in two ways.
A - by using library tools / Rename file from properties (Rename Files from Properties)
There is now the possibility to do "update database to point to new location" wich will basically associate a new file to your existing library entry. If your new files are named in the exact same way as your existing ones, you can use this in conjuction with the option "Find & replace" (e.g find .mp3 replace with .ape). After the update is done you need to sync the library to your new files (technical information) by doing library tools/updates.
Note: this method will associate ALL you existing fields to new files: (number plays, playlists, date imported, etc)
B - by using stacks
IMHO this possibility is much more promising. See a recent post I made on the idea:
http://yabb.jriver.com/interact/index.php?topic=46087.0The idea is here to stack your existing and new files (track by track) and to cascade the library data down the stacks. Using the tip I described (with the CFP trick) you can even choose exactly which data fields you want to transfer. This work well but the current implementation of stacks lacks a few features to make it really an easy process (controlling stack top)
Note; this method will not cascade some fields (number plays, last played, image file) because the CFP trick is not meant for cleaning those fields !
All of the methods I described here are far from a simple command. The later ones don't need any editing outside of MC which is a progress from the mpl playlist method. I think that the stack method could easily be the one that would give us the simplest way to achieve this using the simplest procedure. To this we would need more control on the stack top and a "cascade properties" command much more like CFP but with additionnal fields (number plays, etc..)
In any case have fun !
Best,
Christian