Ouch; so sorry to hear about your loss and frustration. Interesting that they sent you Start 2012 instead of the latest edition, Start 2013... but unfortunately, opening your old 14 files looks really problematic. 😞
See the tables in this Wiki article:
According to the info there, no version of Start Edition can open any professional projects other than AC 8.1 or 9. 😞
And nothing can save in Start format other than Start Edition itself - which can only save to the same version of Start. So I don't see a conversion option at all.
It doesn't look you have any optionto continue your AC 14 work other than getting another AC 14 or higher professional license... My reseller doesn't show the PPU (pay per use) key on his web site anymore, which is what it sounds like you had. I thought I had read that PPU was being discontinued.
The only option I see there that is cheaper than buying a license is a monthly rental option, which is more expensive and less flexible than the PPU.
One of the forum moderators • AC 26 USA and earlier • macOS Ventura 13.2.1, iMac Pro 10-Core, Radeon Pro Vega 64