1. Backup your world.
2. Copy it into the 1.6 folder and play the world. If blocks are gone, it means your configs are different, and you'll need to reallocate some ids. Delete the world, close minecraft, and restore copy the backup into the saves folder.
3. If stuff's gone, then you probably only want the builds that took ages to make (eg: ME systems, multiblock machines, that 50 foot toilet you made out of TiCo blocks, etc.), and any ores that were present in your old world. Find the configs in BOTH modpacks that correspond to the mod stuff you want to restore, find the block/item IDs for the stuff you need, and make sure the new ids are the same as the old ids. (Just the parts you need, not the entire ids in the config). In some mods, the configs will be in exactly the same format, and you will be able to simply copy the entire old config over, but in most cases new items will be added and the configs will differ. Experiment, although remember to back up your world's original copy in case ids screw up.
4. Check for conflicts. Remember, only ores and complex stuff should be configured, any other basic things you had in your world (eg: barrels, macerators and generators) can be manually re - added by cheating them from Creative or NEI.
5. Profit.
