What Greg said, plus, from my experience- you are given a choice to check a box that says "Yes I own a previous copy" during the purchase process- adobe checks really quick to see if you have a previous version (behind the scene stuff) and if you do you typically get $20 knocked off.
The price right now is $99- I would recommend download a trial version, isntall it, use it, and see if the price drops before your 30 day trial expires.
FYI, there is no behind the scenes eligibility check performed when using the purchasing form. You just select License upgrade and the cheaper price is shown. Adobe does not know if you're eligible until you install and enter your previous version serial number.
When you install, you then enter your upgrade serial number and the installer will recognise it as an upgrade serial number then ask you to enter a serial number from a previous qualifying version. If you have not got one, you cannot license or activate the software and it will not work beyond the 30 day trial period.
That said, I have personally always preferred to pay for a Full license each time. Then you get a new licensed version which means a) you only have to enter one serial number when reinstalling (many people lose their earlier version numbers), b) you can sell or give away your previous version.