[CentOS] local yum mirrors and yum.conf.d

if I have a local centos yum mirror, whats the best way of adjusting the 
yum.repos.d/*.repo files to use this?   If I simply edit 
CentOS-Base.repo there stands a chance that a yum update could conflict 
with my changes.


am I better off creating a 'new' .repo and setting the stock one to 
'enable=0' ?