How to expandably obtain pendant of different (to be specified) category of arbitrary explicit character token?

Question

This question is about categories of explicit character tokens.

If you apply \string to an explicit character token, you get the category 12 pendant of that explicit character token.

(Exception: If the character code of the character token is 32, which denotes a space character, then you get the category 10 pendant of that explicit character token.)

So \string can also be seen as a means of expandably obtaining the category-12-pendant of an arbitrary explicit character token (whose character code is not 32).

Are there means for expandably obtaining pendants of other categories of an arbitrary explicit character token?

So that, e.g., you can easily turn an explicit opening curly brace { of category 12 into one of category 1?
So that, e.g., you can easily turn an explicit closing curly brace } of category 12 into one of category 2?
So that, e.g., you can easily turn an explicit hash # of category 6 into one of category 3?

If not: What is the reason for not providing such means?
Would such means allow to run into ambiguities/problems/troubles, probably distorting fundamental concepts of TeX?

(If such means are implemented, probably a test goes along with them for finding out if a token is an explicit character token at all?)

Answer 1

There is ambiguos behavior of categories which are interpreted at token processor level. What happens when you want to have category 0 or 5 or 14 created by your expandable macro?

But you can do this (for other "normal" categories) by LuaTeX extensions:

\def\tokencatcode#1#2{%
   \immediateassignment\edef\tmp{\the\catcode`#1}%
   \immediateassignment\catcode`#1=#2
   \scantextokens\expandafter{\csstring#1}%
   \immediateassignment\catcode`#1=\tmp\space
}

$a\tokencatcode !{7}2 + b^2 = c^2$

\message{.......\tokencatcode !{7}} % yes, it is expandable

\bye

or more simple is to do it with \directlua:

\def\tokencatcode#1#2{\directlua{tex.cprint(#2,"\csstring#1")}}