a new formation with un- has been introduced when that with in- has acquired a connotation which it is desirable to avoid

- For instance if you want to emphasise the word, making an ironic use of opposites? Using a "new" (or little-known) construction such as uncasual works well in that sort of context, I think.

You can also bring in double-negatives to good effect:
"He was not uncasual"