You don't have to subscribe to iTunes Match or pay to upgrade them, you just have to remove the files and re-download them from iTunes.
I would suggest moving them to another directory rather than deleting them, just in case you are unable to restore them from iTunes for whatever reason (removed from the store etc).
Unfortunately, not all files will be restored as a DRM-free AAC track, some are still being distributed with DRM.
For those, the easiest option is to burn them to a CD-RW, and rip the disc.
It's not a lossless conversion but any of the tools I have seen which can strip the DRM from M4P files losslessly are either paid tools or don't seem particularly reputable.