Can a provisional patent application be changed?

Once a provisional application is filed, you cannot add anything to it. However, you can in fact file a subsequent provisional application for any changes or new developments you may want to cover - you just need to pay the filing fee again.
Note, however, if you want to maintain priority for both or all, if there are more than two provisional applications, your one year deadline for filing a non-provisional application or a PCT application, runs from the filing date of the first provisional application. The easiest thing to do is to use your first provisional application as the starting point for your second and/or subsequent provisional applications - just add the additional description and drawings - that way, nothing gets missed or omitted. Then, when you are ready to file your non-provisional or PCT application, you can simply use your latest provisional application as the non-provisional or PCT application.