Defending a Funnel Metric Against a Skeptical Growth Lead

You proposed cutting a signup form from 6 required fields to 3. Two weeks after launch, signups are up 18%. The growth lead is thrilled and wants to present this to the executive team as an unambiguous win. You're less sure.

1. What additional data would you want before agreeing this is an unambiguous win, and why?
2. Describe a specific scenario in which signups rise 18% but the change is actually bad for the business, using the business metric vocabulary from this subject (funnel, activation, retention).
3. How would you propose measuring this properly — which metrics, and over what time window — before the executive presentation?
4. If the data does turn out to show a scenario like the one in (2), what would you recommend doing about the form change itself?