Need advice on pickup schedule change

Been with my ex for 3 years post-divorce and things are pretty cordial. My new job wants me to start earlier on Mondays and Wednesdays which are my pickup days. How do I bring this up without seeming like I'm trying to change our agreement? I don't want her to think I'm backing out of time with the kids because I'm actually trying to make more money for child support. Any tips on how to approach this conversation?