Hmm... this may be a (albeit a little convuluted [EDIT: after writing it out, very convuluted
.. but easier to do than describe]) way to do what you say...
I haven't got an IPod, but I assume the fields you want to keep are "Last Played" and "Number Plays" (although this should work with any field MC can see and store in a file)
Please: make sure you read the whole message before you start
Before you start you must make sure that the fields you want to transfer to the new files are set to "Store in File Tags" under Trees & Views in the options.
Backup the library and whatnot to make sure you don't stuff anything up...
Rerip the songs as usual.
Now, I'm assuming they go to the same place as the old files (eg .mp3's) but just have a different extension (eg .m4a) - if not, you should be able to modify the process below to suit you.
Assuming all files ripped to the same place and have the same filename, just a different extension - delete the recently imported files from the library (note: only delete from library, not to recycle bin).
Go into explorer (this step can't be done in MC... it has to be done behind it's back
) and rename or move the old (eg mp3) files (or delete if you really don't need them anymore... I wouldn't recommend this until testing this whole process.)
Select the files you want to change (I would suggest testing with one file first) and rightclick - select "Library Tools\Find and Replace..."
Click on "Select None", then scroll down to "Filename", check (tick) it and type the old extension (eg .mp3) into the "Find What" box, and the new extension (eg m4a) into the "Replace" box.
Click ok, then click on another file in the list.
This should point the library entry at the new file (it will complain about not updating a file or something, ignore this) and since the old file is 'gone' (renamed in explorer) it won't be able to copy the old file over the new one.
At this stage the tags still refer to it as an mp3 & wont be able to play, that's ok...
Make sure you have the "Update Tags when file info changes" option ticked (under General in the main options) and rickclick on the file\files and select "Library Tools\Update Tags (From Library)"... this should take all the old info and put it in the new file.
Now rightclick the file and select "Library Tools\Update Library (From Tags)" and it should correct the file type and be able to play now, and if everything went right you still have the fields you want in the file (eg "Last Played" and "Number Plays"). Yay!!
Sorry if I explained this the long way, I just wanted to make sure everything was as clear as possible. Hope this works for you... it's actually a bit easier than it seems when you do it. Of course, a automatic way to do this would be great
Tip: An easier way for the file renaming part might be to rip the new files to a different directory, then you can just rename the old directory to have MC "lose" the files, then add another "find & replace" to change the directory too (you will have to do two "find & replace"s.. I can't see a way to do it with one).
Any questions, just let me know