function send_confirmation() - oh frak it you guys have like the most annoying forum ever

ok enough detail in the title for you?...
Back to the topic;

I want to make changes to the confirmation workflow/content. The first impulse is to start hacking away at send_confirmation() in appointments.php, but this will mean taking responsibility for manually merging future upgrades.
I mean I know I can include the custom file in my (child) theme, but that does nothing to the issue of maintaining a private branch.
OTOH I don't want to get involved with the code tree more than I need to...how is custom code usually maintained?

Alex