A word for someone who occupies a position

incumbent

noun

1 The holder of an office or post:
the present incumbent will soon be retiring

[EDIT]
Usage [emphasis added]:

Aron & Nightgale, Incumbent Workers' Views about Lifelong Learning, 1995, p.7

… (2) evaluate the individual labor market impacts of increased education and training among incumbent workers.

Sims, Reforming (transforming?) a Public Human Resource Management Agency, 2010, p.234

a. If an active eligibility list for the new classification exists, the incumbent employee was on the eligibility list, and the incumbent employee met all other eligibility requirements, he or she might be appointed from the eligibility list to the reclassified position.

Smyth, Employed but not Engaged, 2010, p.90

If the old role becomes redundant, and a new role created, then there is no absolute obligation to move the incumbent employee over to the new role. That said, you do have an obligation in any redundancy to consider redeployment opportunities for redundant employees, and in this case that would involve offering the new role to the incumbent employee.

One word which is used is post-holder.

noun
(formal) a person who has a particular job or position

[Collins]

It is likely to fit in the quoted sentence:

All of the relevant post-holders should be promptly contacted and updated.

Functionary

I think this might be the best option. I wonder if the word make sense in a variety of settings (e.g. government, corporate world, military, etc.)

In business you would say stakeholder. Instead of shareholder, which just means people who own stock in the company, stakeholder means anyone who has a stake in the endeavor, including employees, customers, relevant community members etc.

I think you would just use the phrase - current X.

"Our current Vice-President is Tom."

Office holder?

But not every position is considered an office. Go ahead and coin position holder. ;-)

I agree with Peter's comment:

You're asking for a very general word, yet your dissatisfaction with the answers below suggest you have a particular situation in mind. Just be clear about your actual context, and you'll get a better suggestion.

Your template is too general, and doesn't make much sense to me:

All of the relevant _______ should be promptly contacted and updated.

If you are talking about people, how would they be "updated"? I can see that the positions, or information about the positions (e.g. who holds them) might be updated. But by saying "contacted" you clearly are not looking for a words such as positions.

In sum, if you cannot really characterize what this is about (what you are looking for) then you are likely to be disappointed.

This might be too late, but you're probably looking for the word office-bearers.

a person holding a position of authority in an organization