If you create a paragraph style and apply it to each of your 30 text boxes, you will be able to make the format changes you want without going back to the master page.
Once the paragraph style is applied to all your text boxes, all you have to do is make the text formatting changes on one of your posters, and then, with your text cursor still in that box select Redefine Style in your paragraph style pallet menu.
Thanks a lot, that was easy
You're welcome. If you are going to do more InDesign projects, you should definitely read up on Style Sheets. Thre are paragrah styles, character styles, object styles, table and cell styles. Learn to use all of these, and formating and format changes will become much easier.
Look for some InDesign books (I often see InDesign Visual Quick-Start by Sandee Cohen reccomended here) and come back here with any questions.