The only thing that wouldn't run on System 7 for me was the FSDS 2 installer. Note that was version 2. I found a way to take it off my XP computer that worked. If your upgrade maintains the XP registry and all programs, that should be all that's needed. Note that if you have FS9 installed into any Program Files folder, you may now have OS permission issues.
If so, I would do this:
1. Cut the FS9 folder and paste it outside any Program Files folder. I used C:FS/FS2004.
2. Use the free Flight1 Registry Fix program to reset the registry to the new location.
If you are starting over, my way to transfer:
1. Install Fs9 onto your new/updated computer using the folder name and location you want for your final installation (choose Custom). Do NOT install into any Program Files folder! I used C:FS/FS2004.
2. Rename the folder you just installed by adding something like Backup to the folder name.
3. Paste in your old FS9 folder onto your new/updated computer right next to the renamed folder. Now rename your pasted folder to the same name you used when you installed Fs9 in step 1.
4. Re-install any programs/addons that don't work.
Hope this helps,