Don you are going into complexities that you need not concern yourself with. Always make sure you have a full range of whistles..Yes.. as Alison says you can deviate slightly from the key'd whistle you are playing..but whats the point in cross fingering and half notes when all you need to do is change whistles. Now enough of this, and follow what I told you to do the last time..or you'll be staying in over lunch break...practising your cross fingering.

Slainte Colin Ballygally