Sinister Secret is almost identical, except for some creature conversions and treasure. Danger at Dunwater is very, very similar, but more detail has been put into it, and a good mechanical system has been put in place to determine the party's success, rather than DM fiat. I haven't had a chance to go over The Final Enemy yet, so I don't know how much has changed.
Overall, if you've already played the three adventures, you'll get some use out of the book, but probably not as much as you want. Something you could easily do is use the adventures, but put some new twists into them, such as actually having the house be haunted in addition to the smugglers (maybe the ghost wants them out). If you want to move away from the Final Enemy, you could use Danger at Dunwater to be a prepared assualt by the lizardfolk against Saltmarsh (perhaps instigated by the Sea Princes or Brotherhood).