I've been working in Thailand for several years, with a valid work permit.

Each year in January the WP and visa has been extended for another year, the process usually taking a month, a lot of paperwork, and 2 trips to Immigration and Labour Department.

In January 2010 I may need to be outside Thailand on business and holiday.

So my question is how to deal with this potential problem.

Is it possible to renew WP and extension of stay earlier than the last minute ? Has anyone ever done this, or changed the date of annual renewal to sometime more convenient ?

Most immigration offices renew extension of stay within last 30 days of permission to stay, but will also process earlier for valid reasons such as you have. Not sure how early you can renew WP. Ask at your local labour office. You cannot change the annual renewal date unless you apply for renewal of WP for a different time period.