This is not easy but is possible:
Midas has been converted to 1.7.10
For the mods that are common to both packs this will let you preserve their blocks.
You would also want to use the "ungenerate" feature of
World Regeneration Manager to remove the worldgen of any mods you cannot convert with midas to avoid honeycombing your terrain with small dark pockets that will capture all your mob spawns leaving the surface somewhat "peaceful"
If this involves a change in minecraft versions you will need to create a bridging modpack with consists of all the new mod pack mods, but the versions that applied to the existing minecraft version.
1. ungenerate all mods you are going to remove.
2. load and test the world and see that that content has been erased properly.
3. use midas to convert from the old pack on your old mc version to the bridging pack.
4. replace the existing mod pack with the bridging modpack.
5. load and test the world and see that the old mods are gone, the new ones are avialable, and the preserved ones are still in place.
6. remove the bridging modpack and replace it with the final modpack. upgrade the minecraft version.
7. load and test the world to see that the mods have converted their old mc content, to new mc content properly.
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In step 6 - when placing in the final modpack you might be tempted to overwrite the config with the new config. While midas has given you a strong chance at preserving blocks in the world, blind overwriting has a chance to go catastrophically wrong if other types of ids are different between the modpacks: namely biome and dimension ids - these MUST be set in the new config to the old values if different, which could cause problems if your goal is to convert the world such that it can use the new modpack without config changes.