All this means is that you have more up-front planning to do.
You must map the current db structure to a new db structure.
This all takes time of course, and it's your job to communicate to management the extra time involved. To help illustrate your point, make a visual representation of what could happen to the data if this important step is missed.
Given enough time, almost anything can be accomplished. You must make management either give you that time, or decide that the time investment isn't worth the desired outcome of the changes.
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