PowerPoint 2013 - Apply layouts to already existing slides

[Snuggle Bear]

I'm using PowerPoint 2013 and I have a .pptx document that has already existing content, but I read somewhere you have to set the master slide/layouts formatting first before making the slide content if you want the slide formatting to update when you change the master slide. Is it possible to "map" the titles in the already existing slides to the master slide so that when I change the title formatting in the master slide it automatically applies it to the individual slide titles? Do I need to write code? I don't know VBA, but I know some C# and was wondering, if it's not possible to achieve this in PowerPoint itself using the user interface, could I use Visual Studio Tools for Office (VSTO) or something similar to find the titles and somehow attach them to the master slide title?

Also, I need to be able to change certain small portions of text located in different places throughout the document and I want to put a placeholder that can automatically display the text, which I can enter from a central location and have the placeholders automatically display that. I guess I could use master slides or layouts for that but I just want to change individual numbers in the text, so creating master slides or layouts for that would not be the most elegant way to do it.

Thanks.