It doesn't seem like it's possible to downgrade Apache using the /usr/local/directadmin/custombuild/custom_versions.txt file any more.
# cat /usr/local/directadmin/custombuild/custom_versions.txt
apache:2.4.62:
Running /usr/local/directadmin/custombuild/build apache still wants to install Apache 2.4.63
# cat /usr/local/directadmin/custombuild/custom_versions.txt
apache:2.4.62:
Running /usr/local/directadmin/custombuild/build apache still wants to install Apache 2.4.63