Sigma cyclometers with HRM allow you to change the battery yourself. Also, if you are going wireless, look for an HRM that sends the information in digital packets rather than as an analog signal. The digital variety are immune to interference from stray electrical signals.

I have an inexpensive MIO watch with HRM. I have learned how to relax my left arm while touching the buttons with my right hand fingers. I can continue to pedal at the same intensity while waiting for a reading. It works pretty well because most of the time I am trying to hold my HR in a general area, not within 5 points one way or the other.

Good news! "Changeable" is a word, according to my spell checker.