Description of problem: When upgrading Redhat Enterprise 5.9 to Redhat Enterprise 5.10 a new version of atop (1.27-1.el5) updates the prior version (1.26-3.el5). However, starting the new version of atop always fails with a message in the file /var/log/atop/atop.log that says:

existing file /var/log/atop/atop_20140113 has incompatible header
(created by version 1.26 - current version 1.27)



Version-Release number of selected component (if applicable): 1.27-1.el5


How reproducible: Completely - fails every time on an upgrade of a system from Redhat 5.9 to Redhat 5.10


Steps to Reproduce:
1. Do a yum upgrade on any Redhat 5.x system with atop version 1.26 installed
2.
3.

Actual results:
# /sbin/service atop start
Starting atop:                                                     [FAILED]



Expected results:
# /sbin/service atop start
Starting atop:                                                     [OK]


Additional info: A workaround (which is unsatisfactory) is to delete the entire contents of /var/log/atop (or move it somewhere else), remove the new version of atop and reinstall it. atop will then work correctly, but you won't be able to read any of the prior atop raw logs since atop will complain that the format is wrong.