I assume the 15K is a running-only event? Also, you mention a 15K training plan, but we don’t offer plans for that distance. Where did you get it? Once I have this information I can give you a better answer.
Okay, thanks. So, one option is to continue to follow the 10K plan and replace the easy runs with easy swims and rides of equivalent duration and intensity. Then switch to a sprint tri plan for the two weeks between the two events.