Assuming full i90 (MND focus is vital), changing from a BiS crit focus to a BiS det focus is only like 3 pieces I believe; trading Myth (crit focus) body/earring/necklace for Allagan equivalents (det).

Body: 17 det vs 24 crit
Earring: 8 det vs 16 crit
Necklace: 11 det vs 11 crit

Every other slot is pretty clear-cut* for one piece over the other whether you value Det over Crit or vice versa, as long as you value both over Spell Speed (and we're of course assuming you have the good fortune and time to get everything ideally).

*For clarity:
Weapon: Allagan
Head: Myth/Allagan are identical
Body: Allagan if Det, Myth if crit
Hands: Allagan
Legs: Identical
Feet: Myth ideally, but it's 5 PIE vs Allagan's 5 spell speed so not life or death

Belt: Myth
Neck: Myth if crit, Allagan if det
Ear: Myth if crit, Allagan if det
Wrist: Myth
Ring: Both