It wouldn't work completely because there wasn't a transformation by a spell, Marcel would recover from the magic again because it wasn't a spell to undo his "vampirism" as a part was absorbed, the body would return to producing his normal magic, and Klaus would die for that the magic imposed on this type of "updated original" serves to kill Klaus, so assuming that the magic imposed on Marcel's poison acted on Klaus's body, the seven poisons would kill him in a matter of hours.

@JesseSs12 what about a normal vampire let's say Katherine would she become a upgrade vampire

Look, if it's for magic transfer, it's more possible for the magic to affect her body, now if it's in the case of a normal vampire taking Marcel's serum, he will certainly become an updated original like Marcel too, now in the magic exchange from the body of an "upgraded original" to that of an original vampire or regular vampire, both types are subject to death if this is really the side effect of the magical exchange, this is a hypothesis that I imagined is not official, it could happen several situations, the more likely he is to die due to it being a magical transfer to a body that isn't fit for that kind of magic.

I agree with @JesseSs12 This is all speculation, of course, but there is a limit to the magic a body can take. Kol and Rebekah supercharged Vincent/Finn, for instance. Hope could not handle the power or the Hollow and neither could Klaus, not for long. Granted, they could transfer Marcel's power but the new vessel would not tolerate it.

Okay @Sorrow Blue and @JesseSs12

If they siphoned him to the last drop, they’d kill him, sucking up all the dark magical energy in his body and they could transfer that to Hope. So in that sense it’d work. But he’d wake up again, given that it’s neither a spell to kill him (like the spell Bonnie used against Klaus) neither is it a weapon specifically fashioned to kill him (Freya’s dagger), so in that sense it won’t succeed in killing him. They’d ultimately fail.

The only way to put him down using this method of siphoning is if they’d be permanently siphoning him to the last drop. He’d die but would be resurrected almost instantly but die almost instantly as well and the cycle would continue (like it did when Papa Tunde sacrificed Rebekah). But being that it’d giving them endless power, they’d wouldn’t be able to handle it, enough to transfer it to Hope, and even if they transfer it to Hope, it might damage Hope too.

@Sayitasitis2 Is there really a way to kill an original just by channeling or absorbing it? I think it's not possible, Rebekah's case was for the use of black magic, but wasn't informed that she was going to die was that said in the Papa Tunde episode?, she would just be very exhausted and there would be no magic in her body for him to rise again if the circle of salt was not broken, in addition to the point that originals have a much stronger and more intense magic, Papa Tunde only supported the power because he was a user of black magic and in the case of siphoes they are able to absorb any magic, the witch or siphon would have to be very powerful to break immortality to that point no matter how much vampires live only with magic, there is still the possibility of her restoring herself and her body returning to the normal stage, they don't work like vampires that there is a limit to absorbing them, it is possible for the witch to die with that much dark vampire magic in her body, plus the fact that the originals can only be killed or stopped if they use the oak, the cure, or even a spell like the that Bonnie said she knew to kill Klaus are the only alternatives so far and also the Hollow itself, I'll take a look here on the Wiki about this detail because this information was vague, I'm not saying it's untrue ok? but the originals are far superior in this matter of being fully absorbed.

Honestly, I don’t think that would work otherwise Esther would have done that on her kids (absorbing the spell she’d once cast) to turn them into human again.

Let’s note first that a siphoner has an amount of magic they can siphon at once. And The Serum was created using ‘an ugly [type of] magic’ as Vincent once said and for being unexperienced witches, neither Josie nor Lizzie would handle that much magic.

Another thing is Marcel is dead so the only thing keeping him alive is the Serum spell.

(But let’s for once pretend they could transfer the Serum spell to someone else like Davina did to the shackles in season two or they did in season five):

They will need an immortal being to keep all that magic (probably an Original wouldn’t be enough since the spell was created to kill them, so I’m going to Hope) and an extremely powerful source of power to drain from (like a Tribrid, the Ancestral magic itself or two Originals).

Since Siphoners can’t channel, the spell would be impossible to be spelled by them (without mentioning the risk of death by overdose).

Of course I’m assuming the risks and what would happen.

First of all this is one of the most powerful spells in existence, so I think they would die before the "halfway"...

Second of all, due to the nature of the spell it's more likely that this isn't possible... They could siphon till he die, but not death death, cause he will come back to life later