Asking for a friend as I have got good feedback myself,
If staff member has gotten a promotion in work, but no new contract has been signed how does the staff member stand in regards of employee rights for this job role?
Also if someone has taken on a new role, but doesn't feel it is right for them, can they go back and ask to return to their previous role? Or by agreeing to accept the role, do they just have to suck it up and grow to like the role?
A lot of this will depend on the governing jurisdiction, in which country (and/or state) that person is, and on the existing contract.
One trick is to ask those questions yourself to your boss (or the person in charge of those decisions), and then memorialize retroactively what they agreed to by summarizing what they said to you over email (carbon copying HR).
In many cases, this time-stamped written communication can be used in lieu of a contract, especially if it's acknowledged by the person you spoke to as correct in their emailed response back to you. Just make sure to keep a paper copy of those emails at home, for your personal records (preferably with all the headers showing as well).
With that said, if someone is permanently hired to replace you at your previous position, note that your boss may be less willing to hold that spot for you if things do not work out. But in either case, you just need to ask them those questions, and then, see what they say. In the end, it's a negotiation.
Also, if additional benefits are promised, do not be afraid to ask for a new copy of the contract to be drawn up. Ideally, having a real contract on hand would be better than having an email conversation.
I'm not a lawyer, but it doesn't matter cause the answer to this question shouldn't be legal, it about common sense.
And what common sense says is that whenever your work status changes to the better (notice how I bold it when I say better but not worse?), you make sure you sign a new contract specifying exactly what the improvements are (e.g., salary, benefits, days off, whatever).
Depending on laws where you live, customs of the land, quality of legal advice (yours, and the company's. I'm guessing theirs is better), and, ultimately, the judge presiding, you may have a legal case if something is not enacted that was promised verbally as part of the new position/promotion.
Of course, all these factors also play a role if you do have a new contract with all the details written in it, but it makes proving what you were promised, and later denied, a hell of a lot easier.
TL;DR - When your work status changed, even internally within the same company, make sure, demand even, to sign a new contract specifying the new position's details.
