Well, as you should know, when changing a cab from horizontal to vertical you just need to make sure you can rotate the monitor frame 90 degrees and mount it back in securely. So, here are some things to consider:
1) Pick the right cabinet. Some cabinets are easy to rotate the monitor because they manufacturers used the same cabinet and monitor style for different games both horizontal or vertical. Nintendo and Taito cabs are an example of this. If you're not sure, list the cabs you are considering and we can help.
2) Your current monitor mounting brackets may not work when rotating the monitor. Consider a horizontal shelf-mount frame. It has the mounting flanges/holes along the left and right (short) side of the tube. When you rotate the monitor, these will now be on the top and bottom of the tube. If your cab's mounting brackets are on the left and right side, you'll have nothing to mount it to. This can be remedied by installing (or making) different mounting brackets, but it's sometimes a lot of work. If you can swap the tube and chassis to a vertical shelf-mount or universal mount frame, it could make things easier.
3) Another thing to consider is whether the monitor will physically fit inside the cabinet in a vertical position. Some cabs have a small bezel area not much taller than the horizontal height of the tube. Rotating the monitor could mean that an inch or more of the tube may extend above the speaker shelf or below the control panel. Also, depending on the angle of the monitor mount, and the depth of the cab, you may find that the frame sticks out of the back and keeps the back door from going back on. You can always cut a hole in the back door and install a bubble (or wood frame box) around it, but that's more work also, and will cause the cab to sit away from the wall a bit.
Rotating the monitor can be done, but just make sure to consider everything before doing so. Pick the cab with the least amount of work entailed if possible. I've hat to install a vertical monitor in cabs that were originally Asteroids, Stargate, Joust, etc, so it CAN be done. It's just not always easy...