Except for minor changes, amending the specification of a patent application is generally not possible if the patent priority date needs to be maintained. Whether it is a provisional (temporary) or a non-provisional application, the priority date can only be preserved if the specification is not substantially amended with new matter.
What is a patent priority date?
A patent filing date priority date establishes the effective filing date for the purposes of examination of the application and is the first filing date from which the inventor is entitled to exert their patent rights.
For example, if one files a provisional application, then to benefit from it, its filing date would be considered as the priority date for any subsequent utility application filing (a quick refresher on the patent process can be found here).
What is new matter?
Any matter that is not previously described in a patent application can be considered as new matter. This means any information introduced later after the initial filing can be considered as new matter. Once an examiner deems any information in a patent application as new matter, the burden is on the inventor to disprove it. However, convincing a patent examiner that he or she is wrong can become a very expensive affair.
New matter can only be presented in a new patent application filing, whether it is a provisional or a non-provisional application. When new matter is presented and claimed in the patent application, the priority date of the application shifts to the date on which the new filing was submitted to the USPTO.
What can be amended in a patent application?
Minor amendments to the specification as permitted. Also, amendments to the claims and abstract are permitted as long as the amendments are supported by the disclosure or specification. Claims are frequently amended in a patent application when the examiner issues an office action rejection under 35 USC sections 102, 103, and/or 112.