We're still in the dark to make an informed decision but it sounds like you can do one of these two depending on your budget. Top prize and recommended by Goog in your circumstances is to make you site responsive. The transition should be seamless though and not involve creating a new URL structure, bloated cose etc.
On a tighter budget you can copy everything across to a mobile sub-domain (I wouldn't bother with a .mobi extension in 2015) and have a very basic CSS with viewports, 140% font-size, make sure touch points aren't too close etc. Five years ago you might have been penalised for doing that but not these days but if you want to be 100% safe you can read up
here.