a screenshot of your hd would be good using gParted.
If your Windows is ok , leave it there along with efi partition if you have it.
So basically a direct replacement from buntu to popOs should be Ok , i don't know off hand the difference in install size though.You 've also checked which architecture i guess 64 bit?The new install should also re-install grub. Once you've installed new popOs run
sudo update-grub, that will update it and it should include Windows.
For new installs dual boot Windows and linux i always used to install Windows first for 2 reasons :
1) you can do a disk sort out using Windows defrag
2) if you install Windows last its totally selfish , no regard to anything else awill affect boot menu.