TL;DR Make her 6 mana with 400 health and 90 damage, doubled stats on 4th play onward
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Current state, comparison
Her rework is pretty sad. Currently you basically go from a bad 3-cost card three times to a 5.5-cost card. That's not enough of a payout for how much you destroy your gameplay just to get her activated. Her activated form is not even worth playing unless totally necessary, which makes it even worse of a payout, and makes it even harder to activate her.
Her unactivated form is worse than a 3 cost Warrior in a few aspects. Worse damage delivery, lower health. Her activated form is basically an enraged Woodsman, with all the same weaknesses that make such single-body expensive bland cards never played.
Card - Cost - Health - Damage - Attack delay - Cooldown - Speed
Warrior - 3 - 350 - 70 - 0.60 - 2.10 - 5
Demon Warrior - 5 - 300 - 75 - 1.40 - 1.90 - 4
Woodsman - 5 - 600 - 100 - 1.00 - 2.00 - 6
Demon Warrior - 5 - 600 - 150 - 1.40 - 1.90 - 4
Issue
The cost of being obligated to play her is more than just "2 mana" three times. That extra 67% mana over a Warrior comes at a very real cost of tempo, board control, and cycling to better cards. That can be a large amount of XP loss, that the activated form never makes up for. Once you activate a card like this, you should feel good about using it, it should feel reliable and a bit stronger than a card of the same cost. This is not the case for this Demon Warrior. You only get further behind by playing her even in her activated form, since she's so slow to do anything, playing a more expensive card that has better abilities, or a few cheaper cards, will always lead to a better position.
5 mana cards
Cards that cost 5 are sort of problematic. They need to be really worth investing this much mana into it, when so many cheaper cards also have strong abilities, and paying just a bit more can lead to other cards with far higher pure stats. They hurt your cycling, but aren't expensive enough to warrant being massive threats like the old Demon Warrior was. Designing such a cycling card that goes from 3 mana to 5.5 around this difficult amount of 5 mana is rough.
Suggestion to 6 mana
I suggest that she becomes a 6 cost card with enhanced stats, with her base form similar to a Shi-Hou and her activated form becomes similar to a Blue Golem, both very generic yet solid cards for their cost. You would then go from an okay 4-cost card three times to a 7-cost card onward. The heavy loss of tempo early on would then properly be rewarded with a card you can actually use in the late game. This would be a sort of successor to the 6 mana Rock Rivals.
Keeping her at 90 disallows her a few damage thresholds early and later. Slower attack delay means she takes a bit more damage in return. Faster speed and the inability to gain Giant Growth (because we know Blue Golem has been balanced around Giant Growth lolz) means you can't build as much of a push with her, although she reaches units faster. It would feel rewarding to play her in her activated form, but you'd still need to play her right to get value.
This might be too valuable depending how often you can realistically get her out in 1v1 or 2v2. Base health could be 350 or 375 instead, or maybe even on 5th play.
Card - Cost - Health - Damage - Attack delay - Cooldown - Speed
Shi-Hou - 4 - 400 - 75 - 0.90 - 1.80 - 7
Demon Warrior - 6 - 400 - 90 - 1.40 - 1.90 - 4
Blue Golem - 7 - 750 - 200 - 1.10 - 2.20 - 3
Demon Warrior - 6 - 800 - 180 - 1.40 - 1.90 - 4