What Rega do is basically take in your worn cartridge and send you another rebuilt one, your old one is then rebuilt for the next customer. I wouldn’t hold your breath for an original Elys though, it was replaced by the Elys 2 some years ago now.
They’ve never offered a cartridge trade up scheme as far as I’m aware.
If they trade in your old Elys against a rebuild Elys 2, you might be able to trade the new one against an Exact, but you won’t make a huge saving, since you’ll have to pay the rebuild cost (£83 iirc), and a brand new retail Elys 2 is only £125.
If your purple Elys is still in good working order, you might be able to sell it privately for, let’s say £30-£40, and that’s what I’d do.
Finally, I think there are some new Rega products about to launch imminently so I’d just hold fire a bit, if one of those is a new Exact, that would definitely be worth a look... and just one more thing, Rega’s rebuild costs on a higher model cartridge (Exact upwards) are exceptionally good, and unlike the majority of rebuilds, what you get back is exactly to the spec and quality of a brand new one, so they are a sound investment.