1st round of fluid exchange (made simple?)

[spannerrash]

The problem is, you will not be able to put back the same amount as you take out without running the car. If you get 5 litres out you will only get 3 litres ish back in.

If what you mean is to take out the 5 litres ish whilst cold, replace the 3 litres and then start the car and replace the other 2 litres? It's a bit rough but it should be no worse than it already may be. By that I mean if it's low on oil now, then it will still be low on oil afterwards.

If you in intend to do it correctly in a couple of days, why not just do it correctly now. The only thing I can guess is that you want to run around in it for a couple of days to circulate the new oil. I personally don't think it's necessary to drive the car to achieve this. Simply running the engine / gearbox whilst stood still will circulate the oil.

Make sure you do your research before attempting this. It's a high risk procedure if you get it wrong.