The short answer is yes, you will have the opportunity to edit the biography and sources for the final merged profile before it commits to the system.
When you initiate the merge wikitree will ask the person or people on the other profile's trusted list to approve it. If no one on the other trusted list takes any action within 30 days you (or anyone else for that matter) can go ahead and approve the merge. Then you will be presented with the option to select which profile's information to use for each of the usual fields (or even enter new info). At the end you will be presented with the normal text entry field already populated with BOTH profiles' biographies, usually one after the other. Take a few minutes to integrate the text together so you don't end up with two "sources" sections, etc.
So merge without fear! You will be able to keep everything.