The default UEFI bootloader is
\EFI\BOOT\BOOTx64.EFI on your EFI System Partition (ESP).
I have reinstalled Ubuntu last week and it looks like Ubuntu is not creating this file. So Windows cannot overwrite what isn't there. Windows should also not delete entries (registered bootloaders) from the UEFI bootmenu and to my experience it does not.
I would say that UEFI is a major improvement, because bootloaders can coexist on the ESP (if the UEFI firmware, that the manufacturer implemented, isn't functionally broken or crippled).
What you can do to be safe:
- Backup the current contents of your ESP. (Zipping should be fine.)
- To get Ubuntu's GRUB as the default hotpluggable bootloader on your ESP after the Windows 7 installation has finished: copy and create
\EFI\ubuntu\ from your backup as
\EFI\BOOT\ and rename
- Have Ubuntu live media ready so you can use efibootmgr to recreate accidentally deleted bootmenu entries.
I never did a UEFI install of Windows 7. With Windows 8 however it's very easy to create the Windows boot data on the ESP, even after installation. It's basically
bcdboot c:\Windows /l en-gb /s b: /f ALL, but the Windows 7 version doesn't have the