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

Not really. You have a 26mm pivot difference. That's going to wreak havoc with the overhang. 

Thanks!
I thought that there are only 9" or 12" long arms, but I've just found that there is 10" arm (Jelco SA750E). I wonder whether there is a huge market for 10" arm if somehow it cannot be easily tweaked to work on a plinth where 9" arm was used.

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