Operator context
Longer answer
The brand-size threshold for partner-led migration sits around $10M annual revenue. Above $10M, the complexity that typically comes with that revenue (custom fields, B2B, subscriptions, multi-region) exceeds what internal teams reliably handle alongside their normal operations. Brands that try to internal-lead at this size consistently produce longer timelines and worse SEO outcomes than brands that engage a specialist partner.
Below $10M, internal-led is feasible with one important caveat: most brands underprice the SEO and data-quality work. The migration apps look straightforward; the operational discipline around redirect maps, customer comms, and post-launch monitoring is what makes or breaks the outcome. Brands that internal-lead and follow the playbook discipline ship cleanly; brands that internal-lead and treat it as a configuration project usually need recovery work post-launch.
The more useful question is which partner shape. Full-service replatforming agencies (one partner for everything) suit brands without strong internal product or design capacity. Split engagements (data-only boutique + in-house theme team + SEO specialist) suit brands with that internal capacity. Source-platform specialists (deep Magento or WooCommerce experience) suit complex migrations off those specific platforms.
Reference checks at the brand's revenue size are the only signal that consistently works for picking a partner. Ask for one recent migration that went smoothly and one that did not. Talk to both. The patterns the partner names in the difficult-reference conversation reveal how they will handle the engagement; the patterns the partner avoids naming reveal the same thing the other way.