What is discernible to me from your description is that there is change in the constitution of the same entity from that of a partnership firm to that of a Private Limited Company. Therefore, there is no question of transfer of the services of the employees consequent on the change in the constitution of the entity. After the change, the employees of the erstwhile XYZ Technologies would become automatically the employees of the present XYZ Private Ltd., without any break in service or adverse changes in their service conditions. It would be enough that authenticated entries are made in the service records of the existing employees.
Both the companies entity is different. Private Limited Company is registered with ROC and it's business commence from date of incorporate as stated in its incorporation certificate issued by ROC. You can not show transfer of any employee from any other company to the newly formed Private Limited Company unless there is an arrangement approved by ROC in memorandum of artical & association and by High Court.
You need to issue fresh appointment letters to those employees stating their joining date on or after the date of incorporation of the company. However, you can take the burden of previous service of them by protecting their service years in this newly formed company with specific understanding, board resolution and agreement between the company and employees.
As regards to format / drafting the appropriate letter, you need to hire a professional if you can not do it.