Should I keep the old litter tray after buying an automatic box?

Yes, for at least two weeks — a cat with no acceptable alternative does not learn to like the machine, it finds a corner of the house you like less.

Removing the old tray is the most common mistake in the first week.

What you are actually doing

Giving the cat a choice between something familiar and something new. Cats choose the new one surprisingly often when it is not forced on them, and never when it is.

The timeline

Both available for two weeks. Then move the old tray a little further away each week. Only remove it once the machine is being used reliably.

If the cat sticks with the tray

That is information, not failure. Work through the variables — fragrance, substrate, entry height, cycle noise — one at a time, changing one thing a week.

The exception

A kitten or a cat below the machine’s stated detection weight keeps the manual tray indefinitely. That is not a transition, it is the arrangement until the cat is heavy enough.