My question is quite simple, and I guess will start appearing more and more as people use more and more this new feature introduced with 2.9:
I have a running blog with more than 100 articles using the_post_thumbnail. Today I would like to introduce a new page template to my theme, using the_post_thumbnail, but with image sizes different than the ones I have been using today.
It's going to work well with newly written articles, where the new size will be created automatically after I add that new line to my functions.php file, but how do I create new sizes for all the previous articles already published? Is there a way to do it quickly?