1. Go to Tools>File Options>General and select the "Root Person", then under "Start person when opening this database" use "Root person above".
2. Start a new blank database and only enter one person, the home/root person you want, just the name would work. Then drag n drop on top of that root person the copy from the other database. Select the options you want for who to include, but make sure you check the option "Are xxxx and xxxx the same person". This will merge the records when you do the drop.
I tried it that way, and lost data. Not going there again. The Drag and Drop does its work by creating a temporary GEDCOM file, and that's where I lost data.
You didn't mention what data you were missing? Go to Lists>Fact Type List and make sure each fact type in checked to include when "Exporting GEDCOM files". If the option to include is uncheck then that fact will be excluded.
On my own entry (I am the root person), I lost two DNA facts. On others, I lost Miscellaneous facts. On at least one person, I lost one of their two marriages; the spouse had still copied over, but they had no marriage fact.
I ended up restoring from a backup made just before I made the transfer, but it took me almost a month before I even noticed, and I had to keep the two databases and spend more than a week doing comparisons to recapture data I'd entered into the defective copy. I'm still not sure I got it complete.
I don't know ALL the data that failed to come over in the copy, but when I compared the two databases, the new one was more than 10% smaller than the old, and this was after I'd entered new data into it.
For now, I'm just stuck with my great-grandfather's older brother always showing up as the root person in my GEDCOM files.