Most likely the existing electrical box is NOT secure enough for a ceiling fan. I've put about 5 or 6 of these in my houses. In most cases I didn't need to work in the attic at all because i was able to work from below thru a 12" X 12" hole in the drywall. You can buy a fan box at Lowes or Home Depot that has two adjustable rails to fit 16"OC or 24"OC joists/trusses. I reach thru and hammer the lip between the drywall and the joist and use deck screws to mount the plates. If the fan is in the same place as the previous box, the wires will probably reach. If you need to extend, you can use the original box with a cove as a JBOX to extend. When your done mounting, you can cover the big hole with drywall and use a trim ring.