Yeah copying the .tdt files *did*work, but I couldn't see it. Basically when I ran the refresh in the stats tab, it was picking up all the games (they showed as "included" in the info screen) but not showing them in the stats. I figured out it was something to do with the players.
So I probably did this the long way around, but I went into each individual game and clicked on each player and got the "this player is not in your database" warning. I added them (or merged them, whichever was appropriate), and then refreshed the stats tab and all the games appeared
PS I know there's an option to "merge players in current tournament into database" and to add them as well, from the side panel, but this didn't seem to work - this is likely because of my own bad organisation.
Ialao had to go into the database and find the player images and copy them in to the "new" database, and associate them with the players, but once again this was likely down to my own chaotic approach to things lol.
Once again Corey, your help and speedy response is amazing. Really appreciate it.