The solution!

You must go over a temporary CHARACTER SET OCTETS to
let Firebird ignore the character set.

For example in your table you add a third column:
tmpFld VARCHAR(70) character set OCTETS

Then you can first update
set tmpFld=oldFld

followed by
set newFld=tmpFld

Without the additional column, you can use
set newFld = cast (oldFld as VARCHAR(70) character set OCTETS)

Remember, that this will only help letting do
Firebird something, whether this is the right thing
depends on the nature of your input data.

Regards,
Peter Jacobi