You're wrong. 1.6.4 -> 1.7 is NOTHING like 1.2.5 -> 1.3. Most of the changes are related to mapping name changes, the ID removal (which only affects more complex mods) and the network system rewrite.
1.7 -> 1.8 will get a lot more complex, from what I've tried to analyze via the snapshots. I don't think 1.9 will be any bad at that point, though.
1.7 -> 1.8 will get a lot more complex, from what I've tried to analyze via the snapshots. I don't think 1.9 will be any bad at that point, though.