Migration level
⎕ML ⎕EL

Migration level (⎕ML
, ⎕EL
) or Evolution level is a number that, in Dyalog APL and APL+Win respectively, determines exact where the original NARS specification and the current APL2 implementation differ. A lower value represents a behaviour closer to NARS, while a higher the value represents a behaviour closer to APL2.
APL+Win settings
[TBD]
Dyalog settings
Dyalog APL currently offers the following settings:
⎕ML=0

Z←∊R

Monadic ∊ is interpreted as Type rather than Enlist.

⎕ML=1

Default behaviour  
⎕ML≤2

Z←↑R

Monadic ↑ is interpreted as First rather than Mix.

Z←⊃R

Monadic ⊃ is interpreted as Mix rather than First.
 
Z←≡R

Monadic ≡ returns a positive rather than a negative value, if its argument has nonuniform depth.
 
⎕ML=3

R←X⊂[K]Y

Dyadic ⊂ is interpreted as Partition (⊆ ) rather than Partitioned Enclose.

⎕TC

The order of the elements of ⎕TC is the same as in APL2.

Workarounds
When migrating to Dyalog APL, the below dropins for the affected builtins allow setting ⎕ML←1
while preserving code behaviour. It is then safe to programmatically (as shown in the rightmost column) change all occurrences of the affected builtins with their respective covers.
Migrating from  Define  Then run this on all code 

⎕ML=0

Epsilon←{×⎕NC'⍺':⍺∊⍵ ⋄ ⊃0⍴⊂⍵}

'''[^'']+''' '∊'⎕R'&' 'Epsilon'

⎕ML=2

UpArrow←{×⎕NC'⍺':⍺↑⍵ ⋄ ⊃⍵}
RightShoe←{×⎕NC'⍺':⍺↑⍵ ⋄ ↑⍵}
EqualUnderbar←≡

'''[^'']+''' '↑' '⊃' '≡'⎕R'&' 'UpArrow' 'RightShoe' 'EqualUnderbar'

⎕ML=3

UpArrow←{×⎕NC'⍺':⍺↑⍵ ⋄ ⊃⍵}
RightShoe←{×⎕NC'⍺':⍺↑⍵ ⋄ ↑⍵}
EqualUnderbar←(≡)
LeftShoe←{×⎕NC'⍺':⍺⊆⍵ ⋄ ⊂⍵}
quadTC←(⎕UCS 8 13 10)

'''[^'']+''' '↑' '⊃' '≡' '⊂' '⎕TC'⎕R'&' 'UpArrow' 'RightShoe' 'EqualUnderbar' 'LeftShoe' ' quadTC'

Alternatively, the definitions can be used directly inside the text substitutions, whereby one avoids polluting existing code with these cover names.
External links
Documentation
APL builtins [edit]  

Primitive functions  
Scalar  
Monadic  Conjugate ∙ Not ∙ Roll ∙ Type  
Dyadic  Add ∙ Subtract ∙ Equal to (Xnor) ∙ Not Equal to (Xor)  
NonScalar  
Structural  Shape ∙ Reshape ∙ Tally ∙ Depth ∙ Ravel ∙ Reverse ∙ Raze ∙ Mix ∙ Cut (K)  
Selection  Take ∙ Drop ∙ Unique ∙ Identity  
Selector  Interval Index  
Computational  Match ∙ Not Match ∙ Nub Sieve  
Primitive operators  Each  
Quad names  
Arrays  Index origin ∙ Migration level  
Functions  
Operators  
Other  Zilde ∙ High minus ∙ Function axis 