Your migration represents a huge learning curve, both Win 8 and Office 2013 (ribbon).
I agree it is unfortunate that MS has taken a large step back from it's former design strategy of "backwards compatibility". One Key Point to remember is that in-spite of the seemingly massive changes, most of the change is as you noted user interface / skins.
They are evolutionary rather than revolutionary. So, in windows, most of your batch files and scripts will continue to work, maybe with minor tweaks. In Office the core functionality is still there. The addon functionality is another issue. Again, many of
your macros will continue to work. 3rd party stuff, you'll have to talk to the vendors. Unfortunately, with 2013 being so new some may not be ready for it.
The best thing your company can do now is create a custom training program, either in house or with the help of a professional training company.
You have to identify the key pain points and provide a short training program that will get your users up to speed. Make it a half day to full day for each window and Office. Look at training programs that were created for the migration from 2003 to 2007.
Many are available for free on the internet for you to mine concepts from. And most of the concepts still apply, with minor tweaks for UI changes from 2007 to 2013.
If I was setting up this sort of training I would feature 2 things. First is the "Stop Clicking and Exploring Excel 2007 like a Newbie" book. I found it really good for changing my attitude about the ribbon. The other is provide them with one or more
tools that will help them find where commands have moved.
One of the obvious pain points is customizing Office apps. You have 2 options for that, the obvious one of adding buttons to the QAT, AND you can add new groups and tabs to the ribbon(s) (the second one wasn't an option in 2007, a real PITA). Those customizations
can be rolled out with the initial installs, or even retroactively applied.
One more link I didn't provide you with is this one that you may find useful for your 2003 custom toolbars
Use Word 2000/3 Toolbars in Word 2007/2010
http://www.gmayor.com/Toolbars_in_word_2007.htm
Final point. If I had complete control doing a corporate retaining / upgrade course I would plan it in 2 parts. An "intro" course to get them started, with a planned second ("advanced") phase, 3, 6 or 12 months later after they have had a chance to use it
and come up with questions that apply to their day to day work. Give them the opportunity to submit questions, and take common issues from the help desk to design the curriculum for the second phase. Too many managers assume that all users (even non-technical
"average" users) become "instant experts" after being given an intro course to any software. As a result, everywhere I go I see 1% of users doing "advanced stuff" (ie more complex than copy and paste), while the rest are still using their computers like digital
typewriters (using tabs, spaces and paragraphs to layout documents) and glorified calculators. As a result, most of the potential benefits of desk top computers is not being realized. Simply because users don't know how to do much more than press keys. I'm
not advocating that everyone has to be programmers or "macro masters", but there are lots of relatively simple concepts that they can be taught to make their jobs easier.
2010 and 2013 added a lot of functionality to allow "collaboration", things like controlled "work flows" and allowing more than one person to edit a file, even at the same time. 1 key to making this new functionality work is the company has to setup a SharePoint
server. Then IT has to do a bunch of stuff and users have to be made aware of it all ... But it represents a lot of potential time and costs savings for the company, that will probably not be realized.