is it possible to make it so that a elite can pull out a energy sword when you get to close to him while already wielding a plasma rifle like in halo 2?

You could do it through permutations I believe.
(object_set_permutation ELITE "right arm" SWORDPERM)
but I don't think AI encounters count as objects. I think you can use ai_attach to attach an encounter to a biped or something similar, so you could have a biped, attach the PR AI, then wait for the trigger, then detach the PR AI, change the perm, and attach the sword AI.

Scripting never was my strong point and it's been a while so I could be wrong.

you can use encounters as objects, but all that will do is appearance. If you actually want them to change to other weapon, your best option would be OS (because first, you want the distance to them. Next of all, the weapon changing)

(script startup blah
(sleep_until (and (< (objects_distance_to_object (players) (unit (list_get (ai_actors YourEncounter) 0))) 3) (objects_can_see_object (ai_actors YourEncounter (unit (list_get (players) 0)) 90)) 10)
(unit_data_set_integer (unit (list_get (ai_actors YourEncounter) 0)) current_weapon_index 1)
(ai_berserk YourEncounter 1)
)

Without OS, you would need AI that act as a volume trigger to see when your in range, and you would actually have to replace the AI to get one with a sword.