No, you can’t choose how much to exchange. When you opt in, all your eligible EUR Notes are included.
However, you may receive a partial exchange if fewer bonds are left than your holding. The new bonds have a minimum denomination of €100, so a partial exchange is rounded down to the nearest €100. For example, if €1,250 of Bonds can be allocated to you, you’ll receive €1,200 in Bonds. Any amount not exchanged, including the amount left over from rounding, stays invested in your existing Notes.