[Verizon Samsung Galaxy S III] newuser134:
Ok, to those who want to look at the technical side of this, just found another piece of evidence implying that the i535 should be physically capable of operating on UMTS band II (wcdma 1900). Go to this link, go to the UMTS-Frequency Divided Domain table (the first table), rows 2 and 3 show the upload (transmit) and download (receive) paired frequencies of band I and band II.

i) The transmit frequency range for band I is 1920-1980 MHz.

ii) The transmit frequency range for band II is 1850-1910 MHz.

iii) The receive frequency range for band II is 1930-1990 MHz.

Anyone that knows a little about RF transmitters and receivers would know the following:

From comparing i) and ii) it is obvious that since the radio chips are identical msm8960 in both i747 and i535, it would economically unviable to make the chips different in hardware so that one would be physically disabled from transmitting at 1910 MHz when it can transmit at 1920 MHz. We know the i535 operates on wcdma band I, so its lower transmit frequency is 1920 MHz, it would be more expensive than not to make its digitally controlled tuner to physically be disabled below 1920 MHz, and make a slightly different version that can transmit from 1910 MHz and down. If the radio chip in the i535 can transmit on the 1920-1980 range, it can probably, almost certainly, also transmit on the 1850-1910 range since the upper limit of one is so close to the lower limit of the other.

From i) and iii) it can be seen that the transmit range of band I almost entirely overlaps the receive range of band II, 1920-1980 vs. 1930-1990 MHz. People with knowledge about RF transceivers also know that digitally controlled RF transceivers are usually software controlled. If they can receive on a certain frequency range, they can also transmit on the same range, and vice versa, unless disable by software. If the i535 is capable of transmitting data in the range of 1920-1980 MHz with wcdma modulation, and it has the same radio chip that the i747 has, then it can also receive in that same range a signal with the same modulation, hence proving that in fact it can physically receive wcdma signals on band II as well.

These two comparisons above show with very little doubt that the i535 Galaxy S3 has the same physical capabilities to send and receive both wcdma 2100 and wcdma 1900 (band I and II), and the fact that an identical radio chip in the i747 (AT&T version Galaxy S3) can do this as well, should leave almost no doubt about the physical hardware being there. It is disabled by software only.

On the last column of the Frequency Divided Domain for UMTS on the page linked above, it shows where each band is mostly used. It seems that almost the entire world (with a few exceptions) uses wcdma 2100 for 3G/HSPA+ data, but only North America uses wcdma 1900 (even T-mobile has started using it now). So why on earth would a Verizon CDMA/EVDO/LTE phone have all gsm bands AND wcdma 2100, which is used everywhere else in the world but in North America, but then not have wcdma 1900 (that ONE single band) for savings? Why would it need wcdma or gsm at all? (because it was already on the phone's radio chip, and for roaming), why is it missing the ONLY band that other carriers here use for 3G/HSPA+? (because it was deliberately disabled by software/firmware to make this phone incompatible with domestic gsm providers' 3G/HSPA+).

Another thing to notice from the link above is that if you use Netmonitor to poke around in the UMTS band selection menu of the i535, one of the choices is IMT2000 under the wcdma menu. On the page linked, IMT2000 is defined in the text at the beginning of the page for all frequencies ranging for both the 1900 MHz band (II) and 2100 MHz (I), yet when you click on IMT2000 in the UMTS menu of the phone, it only shows wcdma 2100, it must be disabled by software. This is all proof that getting an AT&T (or T-Mobile) modem successfully flashed to the i535 (without bricking it) would enable wcdma 1900 and make it functional on domestic gsm providers' 3G/HSPA+ networks with data.

Please feel free to respond with your thoughts/comments on this.