Hi Liz
My standard answer seems appropriate here: can you safely jump back into upper plank position? Yes! No! Maybe! It depends!
Many students will have no problem jumping back to upper plank. But, many students should be very cautious about doing so.
Many students will have no problem jumping back to lower plank (chaturanga). But, some students should be cautious about even this.
There are training exercises which work with the bouncing movement of falling into a push up and bouncing back up to standing! This is very cool for those trained to do it. (These are called drop push ups.) It requires a high degree of bounciness in your fascia, which can be trained over a long time. So, for the people who can do this, jumping back to any plank variation is totally fine.
The real question should always be, “what is this like for YOU?, If you have shoulder instability or elbow problems, it is probably not a great idea to be jumping into any version of plank-walk yourself back into it, using co-contraction of all the shoulder stabilizers.
The analogy of jumping back to upper plank being the same as landing with straight legs is not quite right. The knees should have some give when landing from a jump, but the legs are not the same as the arms. The shoulders have a lot more mobility and “shock absorbing, capacity than the hips. I have never felt a shock in my elbows from jumping back into upper plank. My shoulders take up a lot of the stress. Plus, my elbows are never locked into full extension either: there is always some microbend and softness there. You don't have to land with elbows greatly flexed to take up that stress, but some bend there with some stiffening around the joint is probably a good idea.
So, once again, “always is always wrong, and never if never right,. As a teacher, be cautious of setting up a
nocebo in your students! Don't use fear as a teaching tool.
What is right for you? I don't know. How do you feel when you do each version? How do you feel when you do the movement, after you complete the movement, and over the next day or two? Any pain or discomfort? If so, it is probably not a good idea for you. But if you feel fine even a few days later, it may well be totally okay for your body.
Cheers
Bernie