Hi, yes it's okay to do that for a short period of time and you will lose weight quicker, as you'd only be getting say x 3shakes plus skimmed milk in your hot drinks around 650-700 cals a day.
The only down-side to reducing from a low cal diet to a very low cal diet (<800cals per day), is that once you re-introduce food again you may either not lose weight that week, or even put a pound or so on, it's just your body's way of adjusting - but that said, if you only do it for a few days that might not happen, I'm just talking from experience and not everyone is me lol!
If you feel this would be good for you and your happy to do it, then by all means give it a go and then you can chalk it up to experience.
Let us know what you decide and how you get on - I'm interested, or is it nosey! x