The initial ones were transitioned in tank, very slowly over the course of several water changes (took two months)
Experience has now shown that to be very, very overkill. Juvenile fish just don’t care. I move them from salt to fresh and vice versa, literally just net and dump. Older fish (showing colors other than black) I’ll put in a small tank and add a cup of the opposite water every 15-30 min (I’m not precise) until full (2 gal tank, starting half full) then I let them have the tank to themselves for a day, mostly just to destress from being netted. Water temp and fish stress are larger factors in my experience. Endlers are very hardy. Fancy guppies I’d be more cautious with as they’re not as hardy as Endlers / wild type guppies.
Edit: Should probably add that the tank has been at this salinity (1.018-1.022) for over a year. On my third generation of fish being born in those conditions, so they’re happily breeding.