I would suggest you to update the tiki version step by step (1.6 to 1.7, 1.7 to 1.8, 1.8 to 1.9), and following all the recommendations in their help pages: such as UpgradeTo18 (and specially this one).
For the rest, you shouldn't have many problems (I did the same as you, upgrading 1.4 to 1.9rc3 for the last two years, sequentially, and it works pretty well right now, The 1.9DR version is pretty functional. You can eve try it yourself through a Knoppix GN/Linux Live-CD with Tiki 1.9DR incorporated! (I did and it's very nice!)
Xavier (tiki novice, and not a real programmer but marine biologist using tiki for teaching at university)