For some this is a very difficult transition to make. Here is the advanced tuck planche:
and here is the straddle planche:
(both pictures are of Jim Bathurst of Beastskills)
The straddle planche is a more difficult move primarily because it creates more torque on the shoulders. You can overcome this by strengthening the shoulders. Some small changes can be made to the adv. tuck planche to make it more challenging:
1) work the advanced tuck planche with fingers pointing back at 45 degrees / \
2) make sure that your shoulders are fully extended and elbows completely locked out
3) keep your lower back fully flat or even a little bit arched
4) start moving the legs further back torward the half-lay position
In fact the straddle planche can be bypased entirely by gradually progressing torwards the half-lay. The half lay is identical to a full planche except you may bend at the knees to reduce torque. So knees bent 180 with the heels almost touching the glutes would be easiest.
There is also the secondary challenge inherent in the staddle planche that is not a factor in the adv. tuck planche or the half lay. To avoid it you may want to bypass the straddle in favor of a gradual transition to the half-lay. The challenge is that the straddle requires hip flexibility and strength that may require seperate focus. Flexibility may be worked with simple straddle stretching but strength may be built with an exercise called youngs:
"This excellent exercise is the most specific for directly increasing your ability to open your hips fully while in a straddle planche. It is also however the most challenging and will be quite difficult to perform unless you already have a straddle L. From a straddle L, and without lifting the hips whatsoever, rotate the legs back behind you until they are together and then return back to the straddle L. During the arc of movement, attempt to very lightly brush the toes along the ground at all times. Once the legs come together, do not allow the back to arch and immediately begin reversing the arc of movement. Intense cramps throughout the hips and glutes are quite common when first beginning to train this movement."
this is from Coach Sommer on Gymnastic Bodies who also has some other suggestions