You've described what I would call a "rewrite", just one that was thoughtfully planned out. Another benefit is that you have the existing, running system, to us

You've described what I would call a "rewrite", just one that was thoughtfully planned out. Another benefit is that you have the existing, running system, to use as test validation. If the new black box works exactly like the old black box, you're good. Of course you need to do a few things like go through an end-of-year with both systems to make sure you found any lingering differences.