Changing the verbiage on the signup form

We have a client who would like to add/change much of the verbiage on the signup form. For example, they feel that just "Username" is not explanatory enough for their potential clients and would like to change this to "Please Choose A Username".

I know I could go hunt down the proper code to do this, but the next time CoursePress is updated (and let's face it, that's often) my code changes would be overwritten. Any ideas? This is a major issue for this client.