Is it acceptable to modify the format when republishing an article?
There are many types of rights related to intellectual property, and it is important to understand them correctly. The one you're dealing with here is copyright, which covers the text, graphics illustrations, and other possible graphic media (video, if your paper include any).

However, the copyright which you transferred to them does only cover these elements. Once they give you permission to reproduce the text and graphics, you can reproduce them in any way you want, following the license. But the presentation (fonts, page layout, etc.) is not part of the copyrighted material.