Ok for attaching the angle braces you want the whole thing as flat as possible so shim under it with blocks or rocks so that the base pipe is as straight as possible and you have enough ground clearance to attach wheels and have them sticking below the ground pipe (it can be checked with a string line or laser level but doesn't necessarily have to be perfectly level with the ground below...just make sure there's enough clearance to move it). Then use either fairly thick walled aluminum pipe with each end flattened (hammer it flat) and predrilled (approx the same diameter as the existing pipe or even a little thinner is ok) or at least 2x2 thick wood (2x4 would be even better but adds more weight) and attach precut angle braces to the posts and base plates. Keep in mind that if you use wood you need to make the two pieces on each side of the post slightly different in size so they don't meet at the same spot where they are attached at the post, or just jog out the bottom a little on one where it attaches to the base pipe. (did that make sense???) After (or before) the angle braces are done you should also add pipe bracing (easiest to clean) across the ground between (right next to) each of the longest base pipes where each post is ....This will keep it from wracking too much and help keep the wheels in alignment, and it would also be a good idea to add angle ground level braces at each of the 4 right angle corners where each base pipe joins to the other... the birds would have to hop over them to get across the pen in all areas though, another reason this would not be best for chicks.
Now, if you want to keep the wheels on permanently (ideal) and not deal with adjusting the height or removing them, after attaching all of the angled post and ground braces while the whole thing is raised up, measure up each post the radius of the wheel, but having the wheel to stick down a bit below the bottom of the base pipe to keep it elevated for moving, yet not enough room for birds to go under it....Then of course you want some sort of rope or chain attached to the front corners for when it needs to be pulled.... uuugh I gotta go make lunch and get other things done but if you need a more detailed explanation or a diagram let me know. Anyway, it would be a bit of work to get that thing movable but it's do-able if you have the time and parts to do it!