copperoutdoorplumbing
My backyard has existing copper pipes outside that I want to reroute. To keep like with like, I'll just continue using copper. Are there any considerations or special types that I need to be aware of? Specifically, is there a special type of pipe that is better suited for outside conditions.
No snow or freezing temps here.
Use a stainless steel hose clamp (sometimes called a worm clamp becasue of the screw it uses) and a couple of L-brackets - clamp the L-brackets to the pipe using the hose clamp, screw them into the wall with wood screws.
The fussiest, hardest to find and most expensive aproach would be to find copper or copper-plated L-brackets ("pipe strap" would be easier to find but less rigid.) 
The practical, less expensive and just as functional approach is to use steel L-brackets and place some durable plastic or rubber material between them and the copper pipe to prevent corrosion. Clamping steel directly to copper will result in galvanic corrosion and an eventual leak.
You can also use stand-off clamps intended for wall mounting on the vertical portion of the pipe. 
The "long-gone plumber" certainly does not appear to have worked to normal plumbing standards...
I did this very thing (replaced copper with PEX for baseboard heating). But I did this because my copper had frozen and split in about 200 places.
So, if your copper is shot, I'd replace with PEX. But if the copper is fine, save the hassle!
But if you do decide to do it, some tips:
Sharkbite connectors are great and make it super easy to connect copper to PEX (at the baseboards)
Be sure to get oxygen barrier PEX. This is designed specifically for heating systems and has a metal liner inside to prevent air from permeating in.
pex is a pain for tight corners. You'll either need room for a gradual bend (they make elbow clamps for PEX to handle radiused corners) or you'll need to use elbow fittings for sharp corners (either those designed for the PEX you are using or, again, sharkbites--though sharkbites can get pricey)
Best Answer
If this is outdoors and possibly underground, understand that there are two types of hard copper pipe (as opposed to soft tubing); type L and type M. Both have the same external diameter so they both use the same fittings, but type M has thinner walls. It is for use where the pipe is going to be somewhat protected, as inside of a stud wall in a building. Type L has thicker walls so the inside diameter is slightly smaller, but it is much more resistant to accidental puncture and will last longer in terms of corrosion when in direct contact with soil. Some jurisdictions require that you use type L outdoors, so it's just better to go that way.