Question: An Incident meets the criteria for P1. However, midway through resolution the impact has changed to that of P2. How should we treat the Incident now? How should we measure the SLA, based on P1 or P2?
Answer: I don’t believe ITIL provides much specific advice about this condition. How you want to handle this is really up to your organization and, more specifically, your Incident Policy.
In general organizations will prioritize Incidents based on the Impact (i.e. how many people or systems are affected) and Urgency (how long the organization can function with that service down or degraded).
Was the original assessment of Impact and Urgency incorrect? In this case you should change the prioritization.
Did the impact change because you applied a fix or workaround? In this case you should not change the prioritization.
Your SLA’s usually measures to full restoration. You could also measure to workaround provided. Your SLA’s won’t usually include fractional measurements based on changes to prioritization. For that matter, most organizations don’t even have formally agreed SLA’s with their customers.