I got it to work. Wasn't sure what it was I was doing wrong initially.
Here's the snippet of what I came up with to apply that mapping to an entire data table:
dt = Current Data Table();
// conversion map to recode symbols
// dot -> triangle symbol
// d -> downarrow symbol
// dd -> double down arrow symbol
// u -> up arrow symbol
// uu -> double up arrow symbol
aa = ["dot" => "◆",
"d" => "↓",
"u" => "↑",
"uu" => "⇈",
"dd" => "⇊"];
// iterate through all columns
For( k = 1, k <= N Cols( dt ), k++,
c = Column( dt, k );
colname = c << get name;
colvals = c << get values;
// Try to change values according to the recoding map
// Try( ) function will not happen if none of the value is not one of the characters to convert to a symbol
For Each( {v, i}, colvals, Try( colvals[i] = aa[v] ) );
c << set values( colvals );
)