1. The date of joining/appointment should be actual date as they joined your firm
2. There should not be material or substantial change/differences w.r.t. the offer terms, existing terms & conditions.
3. You will face a question from all quarters - this means every recipient will be made permanent or there is going to be some permanent and a few as temporary. Find an answer to this.
Are you planning first you circulate this appott.letter in draft form, invite comments/concurrence from all concerned. Based on the such responses, you'll suitably amend the apptt.order and issue. Is't your stand now ? Discuss the impact of this action before hand and be prepared to tackle the developments unfolding in due course.
The common model of your appointment order may start with a first para like this -
We are pleased to regularise the formalities of your employment with us by this formal appointment order.
You are appointed as------------(name of post/designation) w.e.f._________(date of actual joining). Your emoluments are fixed at Rs.________details of which is attached together with other terms & conditions.
You have to modify the above sentences suitably, if any of your employees are promoted or designation changed or transferred to a diff.location as the case may be.
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26th December 2018 From India, Bangalore