One technique is, do it with Custom Fields. Create the custom-field with the Key "Category 1 paragraph" and the Value, type your paragraph.
Then use the WP code for getting a custom field value. It's shown in the Codex or you can use Get Custom Field Values plugin to do it with 1 line of code to insert in your theme (on the single post template). You could do it with Conditionals if you have different categories with a different custom field for each.
This way, each time you need to change that paragraph, just go into a newer post and put a new value for that custom field.
This method will take a bit of work on your part, not quite as simple as a basic plugin. There may be another way.
Another possible thing to look into is the Awesom News plugin. It gives you a replaceable paragraph on your homepage, might be able to use it for single post pages.