What software comes with the $19.99 subscription?

If you're talking about the Creative Cloud Photography Plan, a quick look at Adobe's page about it will tell you what's included: Lightroom and Photoshop | Adobe Creative Cloud Photography plan

(Photoshop and Lightroom)

Secondly, you wouldn't have to uninstall your old version. You can have multiple versions installed.

Hi Randy,

We offer Creative Cloud single for any one application for the pricing of 19.99 USD.

Creative Cloud pricing and membership plans | Adobe Creative Cloud

Regards,

Sheena