They will fit into the box...however if the air space is different between the 2 subs then they will not sound good. Check the specs of the box you have, if they are close to the specs of the new subs then it will work, if not...build a new box.
everyone here is telling you some very good points:
1) the box airspace must be the similiar air space required by the sub.
2) the depth of the sub must be able to fit into the depth of the box.
3) if all the above is good, I would caulk the seams of the box once more or do it the first time(to prevent air leaks).
4) make sure the holes that were created from your last sub are filled in with some wood glue and create a whole new set of holes for your new sub, this is due to the fact that if you reuse the old holes you may cause fatigue from a perhaps heavier sub-frame. Fresh holes will give more stability.