Are Jelco SA-750D and SA-750E swappable?

There's all sorts of arm lengths. 9, 10,11,12,16 etc. They all have unique pivot to spindle distances and must be fitted to align with the requisite measurements. You can't tweak one to fit another or the tracking will be so off as to be unplayable.

To answer your question. No. They are not swappable unless you can increase the P2S distance.

If your using armboards and the armboard is able to be swapped in and out in the EXACT same location than you can swap tonearms with the armboards and preserve your setup adjustments.
Tonearms with different spindle to pivot differences will not mount in the same precise location as has been pointed out. You will need a different armboard for each tonearm.
I designed a system that is precision machined out of aluminum, the main armboard is 3/4" thick aluminum plate that takes armboard inserts made from 1/4" aluminum plate that locate very precisely. I can swap tonearms complete with cartridges and preserve the setup.

BillWojo

I have a 10-in Jelco 750 EB. When I replaced my previous sumiko 9-in tone arm, I had to turn my arm board 180 and drill a new hole for the tonearm.

Many thanks.
I have a plinth that has a hole drilled for 9" arm and I was wondering whether I could fit 750EB, but I guess not without drilling another hole.
Good to know that 10" arm is not uncommon.