TCF - the last post is correct.
we agree that diff pressure is the diff between inside and outside?
Positive at altitude -agreed?
Now descend, leaving the cabin at cruise altitude for the sake of argument - let's say 7000' and let's propose an airtight cabin with no 'valves' open?
As you pass 7000' what is the 'diff'? Answer gone from positive to zero. So "if you haven't increased the cabin rate of descent then surely the cabin differential pressure will decrease;" is correct.
"thus, you'll depressurise earlier than planned. ." is incorrect.
Now descend further what happens to the diff? It gets more negative. Outside pressure is greater than cabin.
What will happen is that the cabin diff will become more negative until the inward relief valve opens - at whatever setting the designer has chosen - then "you'll repressurise faster than planned."
"thus, you'll depressurise earlier than planned." was the mistake. It was just terminology, I'm sure - you meant, I think, "you will reach zero diff earlier than planned". You will in fact be 'repressurising' the cabin which needs to go from 7000' to sea level.