I have to print a map in A0 paper size and also in A3. First I completed the job for the A0 size map, then I duplicated the print composer and I am changing the fonts of attribute tables and label boxes, one by one, to a smaller size for the A3 map. Is there a way to change all the fonts for every text in the print composer, at once? (QGIS 2.18)
Probably a bit late to answer but for next time you can define a project variable "Label_Font" with the name of the desired font, then use data defined property font choice for labeling (with the expression @Label_Font). Once set up when you change the name of the font in your "Label_Font" variable all your label will update with the new font. You can do the same for the label size with another variable.
I haven't tested it so I have no idea if that will work but you could try to open a copy of the .qgs file in a text editor and use search and replace to change the font name, save the file then open it again in qgis