Well you don't actually need to install the upgrade. Deluxe has all the same forms. Deluxe should open your return. Except if you have a state return in your file you need to download the state program first. Except they don't have the updates and states for older than 3 years. So you are stuck.
What do you need to open 2015 for? Do you just need a copy? You could try the TaxPrinter service to convert a .tax file to pdf at
https://www.taxprinter.com
They might be expensive and it is probably not safe to email tax returns back and forth. They are not related to Turbo Tax. I don't know if they can do State.
