Is it safe to Prune Japaneese weeping cherry ?

Yes, it is safe to prune it however, your assumption that it will ruin the shape is incorrect. You should if at all possible always when buying (especially a young tree) select one with a good strong top shoot. After planting and staking, strap as long a cane as you need to the stake to tie this leader to. All the buds that run along this shoot will send out side shoots that will all weep down. When it reaches the required height, you can tip it to stop the height. I have seen so many weeping trees that just look ridiculous (Betula pendula 'Youngii' being a good example) with no height to them and slumping with their branches trailing on the ground. Height gives weeping trees more grace. If you can get it up to around ten feet it will look great. Check the ties periodically so they don't bite into the shoot and after two years, the stem should be fixed in this position and the cane removed.

spring is the best time as it will grow and cover the wound better and quicker .

Thank you Fractal & Noseypotter.Your tips are very helpful,very much appreciated.Fractal,Your comment on height assured me.I am very glad that I was incorrect.Thanks again.

your more than welcome shapla xx.