I'm in the habit of using j to represent the square root of minus 1 (i, for you mathematicians!). Since j isn't used for anything else in Qalculate (I think?), is it possible to allow it to be used interchangably with i? The final display isn't so important.
You could enter
j := i
and a variable "j" will be temporarily saved with the value of "i".
You can also save it permanently by going to Edit > Manage variables > New