Thank you Audisho, I appreciate your help.
Yes I have watched the videos but unfortunately they were not helpful:(
I will elaborate and see if there is something else I may be missing or if you think I should contact support.
I follow the instructions exactly. I begin by exporting a table from the new account so that I have all of the correct field names propagated for me in an excel spreadsheet. I then export from the old site and have an almost identical spreadsheet with all of my activity values in it. Unfortunately, the old site excel sheet has 3 extra fields that the new site does not. No problem, they are empty anyway so I delete the columns and I now have an identical spreadsheet that I can move activities from my old site spreadsheet to my new site spreadsheet. I do this as per the instructions, EXACTLY as the instructions say, and make sure that everything is in the correct columns and then I paste the spreadsheet into the import field with the headings from the new site excel spreadsheet, just as the instructions say.
If you will look at the first picture I posted in the previous post the option _To Existing Field_, as you mentioned in your reply, does not show up for me. Even though I pulled the field names directly from the new site that I am attempting to import into it does not seem to recognize them as the correct field names. After trying to fix this issue for some time I noticed that if I selected _Update existing records by matching key fields_ I could get the option to show up to select_ To Existing Field.
I now understand what you are saying that _Update existing records by matching key fields __will not work. So now I need to know how do I get method to recognize the fields that I am entering so that the _To Existing Field__ option becomes available._
Perhaps it would be best for me to contact support and screen share with them and recapitulate the problem so that they can see it?