I just tore a WiFi only Xoom apart. There are blank solder pads on the circuit board where the necessary parts would go. In other words, it would be a major rework of the device to add those parts, it's not as simple as plugging in the mini PCI card. There are also no cellular antennae in the device. I would suggest there's too much missing to make it upgradeable.

I doubt verizon would design it so upgrading it is possible.

It will probably be possible to swap the "Interconnect Card" with a 4G LTE Module, however the software may be locked to not support it. I cannot verify whether you can or not (I don't own a Xoom) but the teardown makes it look like it may be possible.

I solderd the sockets from an old streak 7 and installed the antennae on to my wifi xoom, i tried a tmobile and a att card and it did not work, i was just looking at the tear down pics again and there are componentand on the MB that are not on my wifi MB, wish i would have look more closely before i did all that work

If they wanted Xoom to have LTE support, they would have included it from the very beginning. I don't think they would release any updated to fix this.

why not add an external 4G dongle? Then you could move it to other computers.

Wifi not working. Cannot connect to internet

Carry one of them 4G hotspot devices in you pocket. I had one from Clear networks. It handled eight devices and was about the size of a pack of smokes. The service was around fifty bucks a month/unlimited. Worked good for me and I did not have to pay twice as much for Verizon's 4G.

i have a xoom and know for a fact that there is no software to use cellular. it would need to be rooted and flashed with a new rom for the original xoom, this might get the software side working.