As a rule-of-thumb you should never modify any standard files that come with a package. So if you want to modify a bibliography style (
.lbx, you name it) it is best to work on a copy of the original file, or even better come up with a file that just amends the original one (a short wrapper that loads the original file internally and throws in some modifications).
Some modifications, however, are very easily dealt with in the preamble of your document (you may still choose to include them in a file to be loaded via
\inlcude or in a
.cbx file, if you'd like to do so).
Enough of that, though: to the problem at hand!
In order to get different results in citations and the bibliography, we need to hook into either the citations and change the command there, or do the same thing for the bibliography.
So you could change the delimiter in every citation via
The standard delimiter will then be used in the bibliography.
You could also do the opposite and redefine the standard delimiters to be
And change the delimiters for the bibliography
For more on these hooks into citations or bibliographies, p. 219 §4.10.6 General Purpose Hooks of the
biblatex documentation might be of interest to you.