I am also often in the same situation.
So for the Op, the same day fare change could be the go, as it say “change fare type”. Book a ticket then call for it to be changed to the right fare for BA maximum earning. I am not sure what would be easier, booking fixed and asking for flex in YBH or booking flex which usually ends up in M and asking for a higher fare, which has no difference in restrictions.
Other option would be of course a travel agent.
Ultimately, you want the cheapest fare that gets 20 TP or work out the up sell to /I/ or /D/.
I think that would be easier than scripts and messing around in the internet.
KF