Power-Utility Dashboard Unit Replacement Button

Build a Replace Unit button for a live power-utility monitoring dashboard. When a field relay is physically swapped out, the replacement serial number must inherit the old unit's location details and its accumulated savings total, so the dashboard continues from where the old relay stopped instead of resetting to zero. Work spans one new DynamoDB table, one new API route, and one React dialog.

The system in short
The dashboard monitors roughly 372 power-factor-correction relays installed on electricity distribution transformers for a state utility in India. Each relay sits in a panel at a substation and reports over a mobile data SIM to an AWS endpoint every ~70 seconds.
One of the dashboard's headline figures is money saved — rupees saved through reduced transmission and distribution losses. That figure is not stored anywhere. It is recalculated on every page load from five cumulative lifetime counters the relay reports: +Ph, -Ph, +Qh, -Qh and CS1h. The counters only ever climb, so the savings figure climbs with them.
That is the problem this job solves. When a relay is physically damaged and swapped for a new one, the new unit's counters start at zero. The old unit's entire accumulated savings disappears from the dashboard the moment the serial number changes, and there is no way to recover it. The utility sees the total drop with no explanation.
So a replacement needs three things to happen at once: the old relay's savings total is preserved, the new serial inherits the old unit's location and officer details (it is the same physical site — only the serial changes), and the unit page then shows previous savings + current savings = total savings.
One detail that makes this tractable: the relay also transmits a daily frame containing those same five counters, and AWS has been storing those daily frames all along. So a relay's end-of-day savings history is already in the database, which is where the preserved figure comes from.