I started to think about how to construct a script and realised that there is a much easier way to correct this.
Step 1: Do a complete backup of your database by your normal method
Step 2: use phpMyAdmin to export your product_description table only to a file, making sure that you tick the "add drop table" and "add of not exists" boxes
Step 3: open the resulting file in a text editor and use the editor's search & replace facility to replace all instances of <div style="text-align: center;"> with <div class="description">
Step 4: re-import the table via phpMyAdmin
Step 5: add .description {text-align:left} to your stylesheet



