I've just found out that the 30mm Dellorto on my scooter is a 4 stroke carb, how easy is it to remove the old spray tube and replace it with the 2 stroke one

To be honest its a bit of a pain, I used to do lots of these conversions in the old days but they get rarer even if you change the tube, you need an atomiser, 2-3 needles, 3-main jets etc so it is not that cheap. If it is a solidly mounted carb don't even think about it just buy a new one its not worth the hassle.

I have a question about "2 stroke spray tube for PHBH (for AV atomiser) Dellorto" have moved all components from my old solid mount phbh-as to phbh-bs(4 stroke) how do i change the emulsion tubes ? thank you

The easy way is to use the old atomiser, file down the head, screw it back in and tap it out towards where the slide would be. Then again do it the opposite way.