Council of Elrond

Unique. Spawn. The Wind-deeps and The Rusted-deeps each have an additional automatic-attack: Spawn-2 strikes with 16/8 prowess/body. In addition, for each Spider attack your opponent faces, you can choose for it to be at +1 prowess and detainment. ...such as Beren fought in the Mountains of Terror in Doriath ... -LotRIV

Can be status non-deatinment changed to detainment between strike sequences (an if not: why?)?

Good question. Offhand, I would say that you must choose whether the attack will be detainment or not when the attack resolves and before strikes are assigned, but I will confess that I have no hard rules to back me up.

I would say, IMHO, that the effect alters an attack. Between strike sequences you are already resolving the attack, so it cannot be altered anymore.

Otherwise you could also cancel an attack between strikes, if the first strike goes badly wrong.

Annotation 13: An attack may not be canceled once its strikes have been assigned. A strike may not be canceled once the dice-roll for the strike has been made.

The same rule does not forbid altering of attack, once its strikes have been assigned.

Okay, didnt know that one.

Still I feel like saying an attack cannot be altered in between strikes becasue of reasons given: a resolving card cant just be changed/altered in any other ways... that would create a weird and illogical situation.

Attack is not a card, it is effect of the card.
Attack cannot be altered before it is resolved.
Before strike assignation (when attack can be canceled) attack is already resolved. Time after strike assignation and between strike sequences is next stage of procces resolving attack.
I realize that changing status from non-deatinment changed to detainment between strike sequences is strange idea.
Question is: is it technically possible (legal)?