Converting IMC to CFC, possible?

I have 20' of 3/8" copper tubing i've been using as an immersion chiller for my 5 gallon batches. However, since i started brewing 10 gallon batches, i need a more effective way of cooling my wort. I decided i wanted to turn my IMC into a counterflow chiller utilizing the copper piping i have used for my IMC.

I'm wondering if the inside of the copper piping will be ok to use to run hot wort through. When i set the IMC inside my wort in the last 15 min of the boil, there has always been water left inside that boils up through the pipe. So i figured there is water sitting inside the tubing while not in use.

What's a good, effective way to clean out the inside of my copper piping to insure my wort wont be infected or exposed to something while using it as a CFC?

Oxy clean, hot oxy if you can. I run a mix of oxy/water at 165F through my system. Also, if you are pumping, pump hot boiling wort through your chiller with no water flowing for 10 mins to sterilize the cfc.

run some pbw or oxiclean through and around(submerse if you don't have a pump) and rinse with no rinse starsan, or homemade nrs(1oz vinegar in 5 gallon of water diluted and then add 1oz bleach), and it will be clean... convering will be fine, but for almost the same money, and ease of use, you could build your immersion chiller into your bk for a heat exchanging coil, so that if you ever get a pump, you could use if for a herms system...