In order to purchase an upgrade edition, you must have one of the qualifying
products listed on the package. If you don't, then you can't install the
product.
When using an upgrade version to change from one version of Access to
another one, you're not getting a new license: you're upgrading your
existing license. That means that legally you need to ensure that the older
version of Access is removed from your machine. When you purchase the full
version, you're getting a new license, meaning you can legally keep both
versions on your machine.
When purchasing an upgrade version when your qualifying version is some
other product other than Access, it's cheaper because they're hoping you'll
become a "permanent" Access user: sort of like drug pushers giving free
samples to get you hooked! <g>