It is typically possible to transfer to another field of engineering if you have already begun Aerospace Engineering, although this might be a difficult process. This will usually be contingent on the institution's policies, and you will likely have to meet certain academic conditions, such as a minimum GPA. Transferring may be accomplished by carrying over credits and satisfying some prerequisite courses for the new field.
It is worth keeping in mind that this changeover can result in a graduation setback as a consequence of the mismatch between course sequences. For instance, if you change to an area such as Civil or Mechanical Engineering, not all your courses in Aerospace Engineering would be transferred into the new program. It is important to consult with academic advisors at an early stage to know the implications, have a smooth transfer of credits, and determine which courses you would have to repeat. In the end, the transition is achievable but needs proper planning and flexibility in your studies.