If I understand you, your father has been sentenced following a conviction and wants to appeal. But without knowing the charge and the guideline calculation it is very difficult to answer you. As a very general rule, most appeals are denied. But successful appeals rarely result in a longer sentence.
This is not a very good answer. For a better one, one that focuses specifically on your father's case, you need to talk to your father's lawyer. Good luck.
The response I have provided is general in nature, and does not create an attorney-client relationship. My practice is based in Rhode Island, and the law and practice in other states or jurisdictions may be different.
There would have to be a resentencing that did not violate the ex post facto clause of the Constitution. Simply, it can happen in only very narrow circumstances in Federal Court.
In most situation an appeal will not increase a given sentence. The viability of the appeal and it's ramifications is best reviewed and researched by an experienced federal criminal defense appellate lawyer.
Of course, every answer or response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. This answer/response should NOT be relied upon to make any legal decisions. Seek the advice of an experienced Federal and/or state criminal defense attorney in your jurisdiction BEFORE you say or do anything.