I do not think you can make these kinds of changes after any attendance sessions have been recorded.
To change it so that Excused gets the same credit as Present, it can be changed in the Site Attendance settings. Under the tab 'Default status set' you can customise the value of each status. When you create a new attendance activity, it will use these values.
I understand that you are already 4 weeks in but you could do the following:
1) Export all attendance data for the course so you don't lose who attended when
2) Create a replacement attendance activity (name it something slightly different to the old one so you don't muddle them up).
3) Create all replacement sessions for your course
4) For each session you have already recorded, you can upload a CSV file to repopulate the entries:
a) Click the button to start the register
b) Click 'Upload attendance by CSV'
c) Drag and drop a csv file with just two fields. I did email and status e.g.
email,status
examplestudent1@thisplace.com,P
examplestudent2@thisplace.com,P
examplestudent3@thisplace.com,A
examplestudent4@thisplace.com,E
Apparently it will except username as an alternative to email but I didn't try that.
5) Delete the old attendance activity