Ok, nevermind, I found the solution myself.

For those interested, once I remove space for all 4 digit numbers, the numbers of 1 000 000 or more look like this: 1000 000. Then I did a Find/replace using wildcards. I did enter:

([0-9]{1})([0-9]{3}) ([0-9]{3})

Note there is a space before the last sequence.

In the Replace with: I did enter

\1 \2 \3

So 1000 000 becomes 1 000 000. Exactly what I was looking for.

Cheers.