From personal experience and the documentation, there may be problems when mixing old and new access control directives in the configuration, e.g.
Order deny,allow Deny from all # ...
Require all denied
Mixing old directives like Order, Allow or Deny with new ones like Require is technically possible but discouraged. mod_access_compat was created to support configurations containing only old directives to facilitate the 2.4 upgrade. Please check the examples below to get a better idea about issues that might arise.
In the following example, mixing old and new directives leads to unexpected results.
I would like to convert to the new directives. It seems like a good idea to do this in one batch and then do a reload. However, some directives are in .htaccess files.