Hello Mike,
A lot of work has gone into our 7.1 to 7.5 upgrade process, so I urge you to please at least test it in your lab to see if it runs smoothly for you or not. Using a copy of the production database along with either a P2V of the SMP or a VM created from the SMP's full server backup (database and server must be from same time period) will give you true results.
If you use AD Import for sites and subnets, then performing the same action on the 7.5 server will achieve that goal. Redirecting the SMA on the site servers will allow them to populate their subnets.
The Replikator Tool (check the KB) will do your software resources and policies.
Manual copy or robocopy, etc will do the physical package files (best to have the same filesystem structure so as not to mess up the package objects).
Hopefully you have placed all of your custom reports, filters, tasks, config policies, etc in their relevant custom folders, so that you can target the folder for export via the ExportImportUtil.