I don't know whether you can make an appontment in KTM earlier or not but if they are taking appointments then you can always cancel it later. However if you are planning to change employer then you only need to provide a copy of the approval notice and your pay stubs. Don't you even have the copy of your approval notice? The employers are required to provide you with the following after your approval comes through;

-  copies of your I-129 forms

- copies of your labor certiification

- copy of your approval notice