Past simple or past continuous for process in the past?

Let's say that I have a computer program that had been doing one thing, but later that program was changed. Which statement would be more correct?

It was sorting by errors, but now it's sorting by warnings.

or

It sorted by errors, but now it sorts by warnings.

And what if didn't have that second part?

It sorted by errors.

This sounds like it was just one time action and not the way that program worked.

Solution 1:

The best is to employ used to:

We use used to + infinitive to talk about a past situation that is no longer true. It tells us that there was a repeated action or state in the past which has now changed. (BBClearnenglish)

So your sentence could be written as:

It used to sort by errors(, but now it sorts by warnings).

The parenthesis can be omitted, and the idea of change still remains. A past habitual action is replaced by a present habitual action, so present tense simple is more indicated in the clause between the parenthesis.