While everything said above is essentially true, depending on the specifics of the boat, it is possible to get away with simply extending your mast a small amount above the jibstay (perhaps something less than 10% of the mast length) and add a longer luff mainsail. I have sailed on a Cal2-30, and gone up on the mast of a less than 30 foot Seafarer both of which this had been done on. I also sailed on a 1960's era Dutch boat that had gone to a fractional rig because of buying a used mast that was made for a fractionally rigged boat. In that case, a short bowsprit was added to balance the increased mainsail area.
Now then in most of these cases, the masts themselves were too stiff to get much of the shift gears on the fly advantage of a boat designed with a modern fractional rig.