How can items from a general self-esteem/efficacy measure be changed to measure context specific self-esteem/efficacy?
It depends on the types of changes you are looking to make. In general, my experience has been that changes to a response scale can be done with minimal threats to validity (e.g. looking to change the 4 item Likert type scale to a 6 item Likert type scale). On the other hand, changing question wording itself usually can't be done "without losing validity" (Goodman, Iervolino, Collishaw, Pickles, &amp; Maughan, 2007).

To answer the "what now" question, you can either use the survey as is, or modify it to better fit your needs (and "revalidate" as needed). This decision is ultimately up to you and should be based on what you think would give you the best information about what you are trying to measure. Based purely off of your brief description and the questions provided, you may prefer to modify the survey. It doesn't seem like the survey, as is, would give you the information you are looking to measure.

Reference

Goodman, R., Iervolino, A. C., Collishaw, S., Pickles, A., &amp; Maughan, B. (2007). Seemingly minor changes to a questionnaire can make a big difference to mean scores: a cautionary tale. Social Psychiatry and Psychiatric Epidemiology, 42(4), 322–327. doi:10.1007/s00127-007-0169-0