I’m only talking about the FTP and W’ graphs in fitness, and I now know they don’t impact anything else.
Use the list view and use the date range for all activities prior to your known correct value.
Tick all activities, then edit to change FTP value. Change it to the lowest value you can think of.
I’ve done that, but I wondered if I could have a non-value instead of the smallest one. I now have it as my first FTP measurement and leave it at that for every value before that.
For wellness fields, you can use ‘-1’ to clear a field.
Don’t know if this will work for FTP, but it’s worth a try…