you can even get more on tds700A series, with 4 resistors (in your case from 744A to 784A only one resistor on acq board) one can get 4GSa/s for single channel and 2GSa/s when 1or 2 and 3 or 4 ch enabled. The bandwidth increase to 1GHz must not work always.
One have of course to let calibrate somewhere (the cheapest way) or did it yourself (TDS700A Field Adjustment Software PN_063260100, NI GPIB controller and some signal gens, counters necessary to do that!).
In principle it does works as well when you burn the calibration data from other TDS784A, but only the missing
data for 4GSa/s not everything, it will not work 100% exact but sufficient. On the TDS700A series the call data is stored
in two X24C02 EEPROMs on bottom side of ACQ board (U1055, U1052). The data can be duped via GPIB, i wrote once
small app to dump that data, see attached code for TDS7x4A models (works similar to tektool from tek forum).
These board ID resistors (R1061<->R1064), whit which the model can be set, are near these EEPROMs as well. Soldered is 0.
TDS784A: 7, 0 1 1 1
TDS754A: 8, 1 0 0 0
TDS744A: 6, 0 1 1 0
interessting that these resistors are not on the TDS500B series schematic (well, they are marked as "exluded from").
on CPU board there are ID resistors as well (R12, R13, R15, R17). As far i remember there was no need to change
them when model was set on ACQ board to TDS784A.
744A - 12,17=0, other 1 0 1 1 0
754A - 17=0, other 1 0 1 1 1
Btw, in principle one could take an TDS540B and change it to TDS784A, sure it will be still no color
(except on external monitor or custom display), but from sampling and bw point of view it would work.
I always liked these TDS700 series and wrked with them for long time, even if they loud and clunky.
And before i forget, similar hacks (with model change) are possible on TDS600B, TDS700C and TDS700D as welll,
but please don't ask me for details, use google to search for pcb pictures and you will find the differences.