I'd probably start first with the vendor and ask questions, this will give you a road-map to develop: Any tools they have for upgrading ... software, database, etc). Is the O/S you have good for the new upgrade (do you need to upgrade to Win 8 from XP). If so, this is a FRESH install ... and would take time installing O/S, Software, Database Conversion ... etc.
If not, then get info on the database upgrade ... will it work as is or do you have to export, map and upload into the new one ? How about current software settings ... does the conversion keep them or do you have to reset everything.
At the Hardware level, ... do they meet the specifications for the new system (CPU, Memory, Video Cards, etc) ... Will it use your network, Routers, VPN's, etc. or is there an investment needed for upgrading.
How much different is the upgrade ... will the users be productive right away or do they have to be trained on the new software.
When will this upgrade take place ... night, weekend ... etc.
Who is scheduled to be there. Systems people, network, users ... How long will it take.
Do you have some users scheduled for testing after the upgrade (make sure the database updated correctly, User Interfaces, etc.)
Do you have User Acceptance documents to be signed off.
How about a Roll-Back plan if it does not go as expected.
Remember to talk to the people who will do the conversion ... get estimated times from each one and system upgrade information so you can calculate time and costs to present to management / stakeholder.
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Start at the top (Vendor), get the information and specs and work down ... talk to developers, DBA's ... anyone there who will do the conversion and create a document of what will be done ... including contact names just in-case.
If ALL FAILS ... do you have an Upgraded Resume ?
Just kidding
Have Fun