You'll have to rinse thoroughly in tap water, and then rinse thoroughly using RO to make sure all the tap water has been rinsed off.
It'll take a while to to do it properly,  butif you have the time then no worries.

Do you think its worth using a few cupfuls of old to reseed as such or just chuck it all?

How do you think my sand dwellers will fair once in the new bed for example sand sifting star and nassarius snails ?

I have a SSSF, when I switched to the larger tank this was one of my concerns,  you can drop or even bury some pellets (use what ever size you think is suitable) in the sand that should help the SSSF, as for the nass snails, they should be fine, but you can give some shrimp to them once a week if you want, just watch them to make sure it's all eaten, but if you have hermits they'll take care of whats left.
Mine is still alive and kicking,  TBH I see less of it now than I did in the smaller tank.