Adding apple juice to the mead will result in a lower ABV than what the mead has now. The main difference between the two methods: by mixing juice in with the mead, the final fermentation will be by yeast that have been stressed in a high ABV environment. How stressed depends on the original gravity of the mead.
When I've made melomels, I've added the honey to the fermented juice. I honestly don't know if the difference would be noticeable.
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