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.