Change `$column-width` at breakpoint, but not number of columns - css

Question

Most Susy examples I've seen seem to handle situations where the amount of columns changes at a given breakpoint, i.e.:
$total-columns: 4;
$large: 12;
// code based on 4 columns
#include at-breakpoint($large) {
// code based on 12 columns
}
But what about when you want the number of columns to remain the same, but (for example) just change the column width? Like this:
$total-columns: 12;
$column-width: 3em;
$large: 64em;
$large-column-width: 3.75em;
#include at-breakpoint($large) {
$column-width: $large-column-width; // automagically changes previously declared column-widths.
}
I would want anything using columns() or span-columns() before the breakpoint to automatically adjust their values to the new column-width without having to redeclare them.
So...
foo { #include span-columns(4, $total-columns); }
would not change at the breakpoint, but the width of the columns would, automatically.
Is this possible?

Answer 1

Susy 2 solves this problem using the new susy-breakpoint with a more powerful shorthand syntax. But the Susy 1 approach isn't much more complex. The at-breakpoint mixin is a simple wrapper for two actions:
Set up a media-query.
Change the global Susy settings inside that media-query block.
I recommend downloading Breakpoint (or another plugin like it) to handle the media-query side of things, but you can also do it by hand. Susy provides a mixin to handle the second part (with-grid-settings). If you put those two together, you have a more powerful version of at-breakpoint.
$large: 64em;
$large-column-width: 3.75em;
#media (min-width: $large) {
#include with-grid-settings($column-width: $large-column-width) {
// new column-width applied to code in here.
}
}