Are they still doing it? They must have extended the opportunity.
I have one of the originals from the first few months they were out. Sent it back when they nicely offered the opportunity to upgrade - who else does that? - and had the full-monty $309 upgrade.
Was it worth it? Probably. Saved me the cost of a new detector. But in the past seven or eight years (has it been that long?!?) I haven't had an inclination to change from the F-75 I had, anyway. I do like the added stability in most conditions. The detector is a LOT less nervous. The additional modes I haven't got full use of - but they expand the potential (cache mode I haven't played much with yet). The boost mode is nice . . . but I still find myself mostly in default mode.
Would I have traded/sold my old F-75 to buy a new one? No. But upgrading a detector I know and really like to make it that much better made sense. I went and treated myself to a new NEL Sharpshooter coil to celebrate!
If you like your F-75 now it is only going to make it better.