Thanks! Just viewed your two-part video on youtube on the swap from 21 to 23. Very nice.
The 21" to 17" seems to always be possible, as they're both 16 kV. My concern is always the 18/20 kV vs. 16 kV limit.
But I ordered a B&K PR-28A high voltage probe yesterday to assist in determining if this mod is sane. Should arrive sometime next week, so we'll see