Can someone in 5D travel back to 4 or 3D .... yes
Can one in 4D travel to 3D ... yes
Can one in 3D travel to 4D .. yes
Can a 3D travel to 4D ... yes but it's, or can be "harder" if only on the surface; and only if one allows it ?

3 - 5 are fluid; they fluctuate since they all have degrees

So, is it fair to say that while one is technically 5D that they are not exclusively in 5, but 5 is their desired Dimension of being & is where the majority of their consciousness, knowledge, etc resides in ... yes

There isn't necessary lower or higher D's, but an overall desired state

But when fluctuation does not occur, and one wants to stay on a lower state (Especially Pride / 175 and below then there is an issue.

This can be true if one stays only one state of 4 or 5 constantly; grow also does not occur because of, again, stagnation.

So some type of fluid nature is, to a degree, is necessary: to allow necessary: learn, love, grow, understand, compassion .....