I remember that changing to an auto-entered calculation from a calculation field does not lose the data from the calc fields results. I tested it and seems as though I remember correctly.
You're right, thanks!
The other day, I thought I had done the same thing (because, like you, I also remembered that changing kept the data), but lost the data somehow. That time, I must have done something else in between... ? Not sure.
Anyway, glad it works the way you remembered!