When changing the innodb_log_file_size variable in my.cnf, first shut down the server, then rename your current ib_logfiles (you can just rename them with .BAK extensions. Then, make the configuration variable change in my.cnf. Then restart server. At this point, you can delete the old backup files if you need to or archive them away.
The reason you need to rename the current files is because, as the error log states, InnoDB expected the file to be 100MB, but it was 10MB. By renaming the ib_logfiles, you force InnoDB to create a new log file at your new specified size.
Since you stopped the server gracefully, and restarted, the new logfiles will contain all instructions InnoDB would need to restore your database back to a consistent state should another outage occur. This is why the old ib_logfiles can be chucked or archived.
Does this answer all your questions?
Jay Pipes
Community Relations Manager, North America, MySQL Inc.
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