You could create one poster and select all the elements which will be common to all posters. Turn that into a Symbol by dragging them into the Symbols panel.
Then edit the artboards and Alt+click+drag to create duplicate artboards. At that point you can then edit/add text or elements not common to each poster.
Now, if you make a change to the Symbol, the changes will update in all the posters. Elements not within the Symbol will not update.
Here's a rough example showing the concept. Here the artwork and titles are the common content (contained in the Symbol). The text for the cities is not within the Symbol. Double clicking one of the Symbols to edit it enables you to change the font and colour or change the layout etc. Exit the Symbol by double clicking outside it, and all instances of the Symbol will update on the other artboards.