You can use the following JavaScript code and execute it in the console to change the font in all text fields for Arial, for example:
for (var i = 0; i < this.numFields; i++) {
var fname = this.getNthFieldName(i);
if (this.getField(fname).type == "text") {
this.getField(fname).textFont = "Arial";
}
}
To know how to use the JavaScript console, take a look at this tutorial by Thom Parker:
http://acrobatusers.com/tutorials/javascript_console