Yup. Let's look at the older FI-15:
http://www.audiophonics.fr/images2/4609_FURUTECH-FI-15_2.jpg
The functional difference is the wire clamping pieces. You can use it, but it's more difficult to get the wires clamped in and the mechanism may be difficult to support higher gauges, like 12 awg.
Looking at the new FI-15 Plus:
http://www.phileweb.com/editor/serendipity/uploads/fi15pb.jpg
http://joshinweb.jp/contents/av/Hiend/FI-15-1.jpg
You can see that the wire clamping mechanism is the same as the rest of the more expensive Furutech connectors. Also, Furutech has stated that the rhodium plating process on this new "Plus" connector is better as well.