Thanks for the reply, Craig. I found a solution that is exactly as you have suggested today.
In eBay's Selling Manager I selected the 70+ items that I wanted to change. Then, I used Bulk Editor > Edit Item Description. When the next menu popped up I changed the drop down to use the "Find and Replace" function. I used this to alter the existing text to reflect eBay's new policy and remove the old one.
As a warning to people that want to follow my example: this method has to use the exact text in the "Find" section to replace.
Thanks a million.