jeez... It could go smoothly or not at all.

If you're talking going from XP to Windoz 10 it could be very bad with older software.

At the very least I'd setup a new computer as an observer and get all the software installed and running perfectly in observation mode before I completed the proposal.

Keith Cress
kcress - http://www.flaminsystems.com

Its probably a reasonable expectation that despite being 'just a hardware upgrade' that an attempt to load older operating systems on it will result in driver issues, as the newer hardware will likely not be supported by the older operating system. Newer operating system may not work that well with legacy software either.
If that's the case then itsmoked's suggestions are a good idea, although its never a good idea to attempt to verify updates and changes on production systems anyway.

I would do a test box like smoke said. Get your standard hardware and all the software loaded up and stick it right besides one that is being used in plant. Leave on floor for a month and ask people to use it first if they have problems have them write a ticket report either thru your system or create a work file on that drive to hold all problems. Then they can use the older workstation right beside it. Then with all ticket problems review the system for improvement, when you think its good enough implement plant wide.