Hi Stuart,
Thank you for your interest in Syncfusion products.
We have created a sample to illustrate your requirement to replace text with formatted text range in the Word document using DocIO. For your reference we have attached the sample for the same. Please do find the sample from the attachment. Try the attached sample and let us know if this helps you.
Please let us know if you have any other questions.
Regards,
Gunasekaran
Sample_13a7a648.zip