I know it's late but I'm working early and would love some help during the day tomorrow 
We have promoted an employee who has been with us for 10+ years.
I've been asked to draft an offer letter changing his salary & job title, I questioned whether it should be a change in terms and conditions letter instead of an offer letter but told it needs to include the probation clause and that the employee, if unsuccessful in the probation period will return t his original role.
Is this right? We will hire a replacement for the role he has been promoted from thus filling our budget for staff (his promotion is replacing a leaver)
I suppose my question is, if you get promoted do you have the right to return to your previous role even though that role is now filled with no room or need for an extra person and should this be a new offer letter or a change in T&Cs?
Thank you in advance.