Short code vs long code when migrating a small Twilio SMS app without downtime

We run a small Twilio-based SMS service (a few long-code numbers, roughly 2k messages/day) and are planning a move to an alternate carrier. The hard constraint is zero downtime: inbound and outbound messages must keep flowing during the cutover.

The open decision is whether to keep using long codes on the new carrier or take the opportunity to move to a dedicated short code. My understanding is that short codes require carrier approval with lead times of weeks, which conflicts with a fast, no-downtime migration, while long codes can be ported but number porting itself may take 1–5 business days with uncertain inbound routing during that window.

We also rely on Twilio webhook retries today, and I'm unsure how to avoid duplicate deliveries if both platforms are briefly live in parallel during the transition.

Given a hard no-downtime requirement and low message volume:

• Is a long-code port the only realistic path, or is there a documented way to stage a short code in parallel without interrupting current traffic?
• During the porting window, how should inbound routing be verified so no messages are silently dropped between carriers?
• What webhook retry configuration on the Twilio side minimizes duplicate sends once the new carrier goes live?