No, the changes you make to your illustrations in Pastela after posting them to pixiv do not automatically reflect in your posts.
To update your posts, please use Pastela's re-upload feature.
To learn more about re-uploading your artwork to pixiv from Pastela, please see below.
How can I re-upload my artwork to pixiv from Pastela?
To use the re-upload feature, you need a subscription to either Pastela Plus or pixiv Premium.
What is Pastela Plus?
What is pixiv Premium?