The checkbox takes a boolean, either as a truthy or falsy.
Airtable will take your 1 or 0 and use type coercion to convert it into a boolean.
The same is not true for converting booleans to numbers.
That’s why you’re not getting any values, as Airtable won’t coerce a boolean into a number value, even though 1 & 0 sometimes represent a boolean.
I think that’s where the confusion comes from.
I’m not too sure exactly how you’re trying to convert your field, but here’s what I would do if I needed to convert a checkbox field to a number field.
Simply group or filter your records based on the checkbox value.
And then just flat out paste a 1 into the number field for that entire group/view.
Do the same for the falsy group, then just delete your checkbox field.