how to issue appointment letter to employees who have joined company 10 years back

One company has issued offer letter to their employees on the day of joining. Now company want to issue appointment letter to their employees to their old employees who has already completed 5 to 9 years with the company.
What are the points to be taken can by company while issue appointment letters to their old employees?
1) what date to be mentioned on appointment letter? (date of issue)

My opinion this letter to be typed to- day date with following condition as confirmations.
This is to confirm your appointment and the continuity of service you have had with us, from ……… as…….. after issue of offer of appointment dated …………
We are now regularizing the terms and a condition of your appointment including salary details, with us as follows, as being done to other employees of the organization:
Salary details
Role and responsibilities
Code of conduct
Confirm ack

Hello,
Rightly said by Mr. Babu Alexander above.
You can also re-word the letter as -
"This refers to your application and subsequent interviews you had with us. With respect to your joining at .................. (Company name) on ........................... (original joining date), we are pleased to issue this appointment letter to you for the post of ........................
Company is in the process of regularizing all the appointments and hence this letter. Your Job Profile is enclosed with this letter as Annexure 1.
Your appointment is subject to the following terms and conditions:-
..................................
..................................
.................................."
Now few important points -
1. Let the date of appointment letter be today's date (current date)
2. All the terms and conditions of appointment to be mentioned as a formal information and not as he /she has already known or done with it. e.g. probation period etc.
3. Mention the Joining salary in the consolidated figure rather than a break up, since your employees' salaries might have drastically changed in last 4 - 5 years. e.g. You will be paid a consolidated salary of Rs. .................. (joining salary per annum).
Hope it helps.

Dear Aniket,
Beyond agreeing on the version stated by senior colleagues here, any appointment letter is a formal document executed by both employee and employer for the terms and conditions if the employment which binds both of them.
Why such laxity or delay in issuing such important document? Please speak to concerned on this issue if your company is doing such thing or else get it approached through the concerned in such a company.

Review the following aspects:
a) Have you complied with the applicable laws w.e.f. the dates of joining of employees? (e.g. depositing the PF, providing paid leave, accumulating it as per permissible limit and allowing its encashment, making provision for gratuity...). You need to be on the right side of law and so while rectifying your employee records you need to examine this as well.
b) Are you planning to insert any service terms/conditions that were not applicable till now (e.g. altering the notice period for termination of service by either side)? Would the proposed alteration call for notice of change/amendment of certified standing orders (if any)/consent by concerned employees?