An easy way (in LibreOffice Calc 188.8.131.52) is not to attempt directly to overwrite what is already present but to create the desired results with a formula in a different column, then replace the contents of the original column with the values (not formulae) from the added column.
Assuming the data is in ColumnA, insert a new ColumnB and in B1 and copied down(†) to suit:
then select ColumnB, Edit > Copy, select ColumnA, Edit > Paste Only > Paste Only Numbers. Now ColumnB may be deleted.
The sheet may be saved with an
† Double-clicking the fill handle won't work here (because of the blank cells). Instead either drag down to suit or, if very many cells, go to the last cell to be populated and enter something there (say
x) then return to B1, Copy, select B2, Ctrl+Shift+Down and Yes.