How do you migrate safely from root to subdomain sending?

Root to subdomain migration process:

Setup:

Create subdomain DNS records

Configure SPF, DKIM, DMARC for subdomain

Set up ESP to send from subdomain

Test authentication thoroughly

Warmup:

Begin subdomain warmup with small volume

Use most engaged segments

Follow normal warmup schedule

Traffic migration:

Gradually shift traffic from root to subdomain

Start with certain campaign types

Monitor subdomain performance

Completion:

All applicable traffic on subdomain

Root domain reserved for transactional or deprecated

Update any customer-facing references

Opening a new office while gradually transferring operations from the old one.