Once you make the change, you have to mouse click out of the field to see the change. If you have Adobe Acrobat Professional (and some versions of standard) you will be able save the document with changes. (The free Acrobat Reader can allow you to view, edit, and print PDF forms but not save them). With Acrobat Professional, you can Save a form in progress and finish it later. If you do not you can:

Print the document as a PDF but you will have to make sure the paper size in advance print options is changed to custom and sized to the dimensions of the poster - 19x22, 22x22, 24x24, 24x36 etc. A free  PDF writer driver is available at CutePDF  and Adobe (this lets you print PDF from their site but only the first 5 documents are free).

Or you can have Kinko's (or who ever your printer is) make the change to the field prior to printing it. If you do this make sure that the printer is aware and agrees to do this.

CutePDF Form filler is a cheaper version of Acrobat Standard (Acrobat Standard and Professional runs $299+).  CutePDF is usually about $29.99.

Here is a sample of the customizable poster. 

Packed in this gathering spot is the following: