Bit Legend:
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MD_CANMOVE      | 0x001 |    1
MD_LOOTER       | 0x002 |    2
MD_AGGRESSIVE   | 0x004 |    4
MD_ASSIST       | 0x008 |    8
MD_CASTSENSOR   | 0x010 |   16
MD_BOSS         | 0x020 |   32
MD_PLANT        | 0x040 |   64
MD_CANATTACK    | 0x080 |  128
MD_DETECTOR     | 0x100 |  256
MD_CHANGETARGET | 0x200 |  512 //removed, not needed
MD_CHANGECHASE  | 0x400 | 1024
MD_ANGRY        | 0x800 | 2048

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1: Can Move.
2: looter.
4: Aggressive: normal aggressive mob, will look for a close-by player and charge him until death. Aggressive mobs also can change target if they are hit by someone else while chasing someone else.
8: Assist
16: Cast Sensor: If mob is aggressive, it will also change targets while chasing/following someone else, otherwise, it'll also be cast-sensitive while idle/random-walking.
32: Boss. Special flag which makes mobs inmune to certain stuff like status-changes and basilica.
64: Plant. Always receives 1 damage from stuff.
128: Can Attack. Everyone with a few exceptions has this. Only prevents normal attacks, not skills.
256: Detector: can see hidden players. Insects, Demons, and Bosses automatically receive this.
1024: ChangeChase: When the mob is chasing a player and it passes nearby another player, it will switch targets and hit this other player. Note this only applies if the new player is withing attack range, it won't change it's chase-path to another player!
2048: Angry: These mobs are "hyper-active". They will change to the closest player while they are following up, They also have skill states "follow" and "angry" (instead of "chase"/"attack") when they pick a new target by their own (ie: not when they change targets due to being attacked). If attacked even once, they loose their Angry mode.

Change Target: (coded)
Only Assist, Angry or Aggressive+CastSensor mobs can change target while attacking. Target must be no more than 3 cells away.
Mob in Hyper-active mode can change target unconditionally.
Mob that chases player can change target only, when he's Aggresive and get attacked.

Old -> New
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64 -> 64: Plants
128 -> 0: Passive immobile beings (pupa) 
129 -> 129: Normal enemies
131 -> 131: looters (porings) 
132 -> 132: Immobile attackers (hydra)
133 -> 133: Aggressive
137 -> 137: Assists others (wolves)
139 -> 139: Assists & loots (thief bugs)
141 -> 141: Aggressive & Assists (no idea how these would differ from plain aggressive, but there's a lot of enemies defined this way)
145 -> 145: Detectors (Giearth) 
149 -> 149: Aggressive & Detector (Hunter Fly)
159 -> 159: Cast sensor, Assist, Agressive, Looter
171 -> 171: Boss, summons, assists & loots (Golden Thief Bug) 
181 -> 1461: Boss, summons, detector & aggressive (pretty much every MVP and miniboss) 
none -> 2181 : Hyper-active (Familiar)