To create a sales order from an existing estimate, use the transform function in the REST API:
POST {{REST_SERVICES}}/record/v1/estimate/{{ESTIMATE_ID}}/!transform/salesOrder
This function takes an estimate with an ID of {{ESTIMATE_ID}} and transforms it into a sales order. You can also specify in the body of the request which fields you want to change during the transformation.
POST {{REST_SERVICES}}/record/v1/estimate/{{ESTIMATE_ID}}/!transform/salesOrder
{
"memo": "Transformed Estimate to Sales Order!"
}