Another approach. This will shield arguments to Hold from replacement attempts, on any level:

line1Points = {{1, 4}, {10, 4}};
line2Points = {{1, 5}, {10, 5}}
xRange = {-1, 11};
yRange = {-1, 7};
arrowPoints = {{7, 4.5}, {7 - Cos[Pi/4], 3.5 + Sin[Pi/4]}, {6, 3.5}};
Graphics[{RGBColor["RoyalBlue"],
  Line[{line1Points, line2Points}],
  Red, Arrowheads[0.05],
  Arrow[BezierCurve[arrowPoints]]},
 PlotRange -> {xRange, yRange},
 AspectRatio -> Automatic]

replaceAllNotHeld[expr_, reprules_] := Module[
    {allPos, heldPos, repPos},
    allPos = Position[expr, _];
    heldPos = Position[expr, Hold] /. 0 -> __;
    repPos = Intersection @@ (DeleteCases[allPos, #] & /@ heldPos);
    MapAt[Replace[#, reprules] &, expr, repPos]
  ]

Example:

In[2]:= testExpr1 = {Hold@"A", "B", {Hold@"C", "D", Text[Hold@"E", "F"]}}    
Out[2]= {Hold["A"], "B", {Hold["C"], "D", Text[Hold["E"], "F"]}}

In[3]:= replaceAllNotHeld[testExpr1, x_String :> LetterNumber@x] // ReleaseHold  
Out[3]= {"A", 2, {"C", 4, Text["E", 6]}}

Gianluca,

The problem with that approach is that the Text statements are generally embedded in a larger set of Graphics primitives that will have many substitutions from assoc. So the first argument will still be subject to replacement. Otherwise the assoc replacement would have to be seeded in at many individual places and the whole purpose of simplified transformation with a single use of assoc would be negated.

Hi David,

glad that you like my approach!

I was never aware of that particular usage of a Module

Well, this idea of course is not mine, I found it years ago in a contribution made by @Leonid Shifrin (if I remember correctly), so in this respect all credits should go to him! But it is a nice way to give "memory" to a function i.e. mimic static variables.

@Neil Singer: I like your solution - and I have the impression that it is more robust. Thanks for sharing!

Regards -- Henrik

Thanks Hans,

That doesn't quite work because the expression I'm doing the replacement on is actually an extended List of graphics directives so replacements are needed at various levels. I would like to have just one replace command.

I think what I have to do is make certain that none of the Keys in the Association are the same as labels I want in a diagram..

Neil,

I have an Association that has a number of rules that represent points or lines or structures in a geometric diagram, or derivation or proof. The right hand sides are symbolic vector expressions with symbolic basis vectors. I use Strings as Keys because I want to be free to use the Keys in expressions using contextual names and then substitute with the Association to do the calculation. I use Associations because I can gather all the data together for one topic. Here I use just one rule.

assoc = Association["A" -> GrassmannPointExpression]

There's also a command that will convert the GrassmannPointExpression to a Mathematica List that is applied after the Association.

But in the following, which might be used in Graphics, the first "A" is intended to be text and not substituted, and the second "A" will be converted to a position. I would like to shield the first "A" and then un-shield it after the assoc substitution.

Text["A", "A"] /. assoc

I would like something like the following. HoldAll does not stop replacement.

Text[HoldReplacement["A"], "A"] /. assoc // ReleaseHold