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

I see that the only difference between these two arms are their effective lengths (229mm vs 255mm).
Can one be swapped with another without changing adjustment?

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

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. 
I own a 9, 10.5 and a 12" from Jelco.. The 10.5, won’t fit on a Thoren 121 or 124. The bottom extend right in the middle of the edge of the chassis.

It won’t fit on the inside, and not long enough to anchor outside.
You have to anchor it to the plinth or in a separate pod...

Good arms though.
If you've never drilled an armboard hole before, it can be a daunting experience. But it's pretty easy once you get the measurements all correct. Let me know if you would want help
Thanks for your help.
My plinth already has two holes, for 9" arm and 12" arm. I don't think I would have another hole. ;-)