While Loop To Replace Text - How To Use Set Processing

I’ve created a script which will do the following.

Update fields in a database which contain instances of an order number. An order number is defined as a 10 digit numeric sting which beings with 998. The first 3 digits of the order number need to change from 998 to 999. There are two types of fields to update: document number fields which may contain 1 instance of the order number and free text fields which may contain multiple instances of an order number.

I created a function which accepts the text to be updated, the text to find and the text to replace it with. The function then loops through the sting for instances of 998. For each instance it finds it checks to ensure that it is at the start of a 10 digit string which contains only numeric numbers – if this is the case then it will update the 998 to be 999.
If the field is free text then it will continue to loop through the string (it will skip forward 10 digits for every order number found) until the end. If the field is not free text then it will exit the script after the first instance found which matches the above criteria, if any.

Need achieving this via set processing and not having to loop through every line. I’ve included the function below and some test data.

--Create the function we will call in order to do the replace
if object_id(N'OrderReplace',N'FN') is not null
drop function dbo.OrderReplace;
go
create function dbo.OrderReplace (
@Textnvarchar(4000),

[code]....