I've done the same thing (for weight, not strength). Luckily, the arrows were long enough that I could just cut them off behind the inserts that were in them (to the overall arrow length I wanted) and glue in my brass inserts.
Now, if you don't know how they'll tune with the heavier brass inserts, you don't want to cut them off and end up with an arrow that's too short/stiff. In that case, you might want to sacrifice one arrow, i.e., use heat to remove the insert as described earlier. It may work without damaging the arrow, but even if you do damage/weaken the end of the shaft, you will be able to tune by further shortening it in short increments. If you ultimately end up cutting off more then the insert length to get a good tune, then you can cut all the rest of them to length. Hope that makes sense.