100% it is ok! The copper itself, could almost careless what is flowing through it. The components attached however, do care, and may suffer, if precautions are not taken. Its all about CLEAN, DRY and TIGHT, to help keep that new system performing, and you from having to field harsh phone calls from the homeowner, wondering why the new system is down already. 

1) Make sure the line size is compatible for your application. There are manufacture charts available for this

2) Visual inspection, make sure there are no kinks or physical damage. 

3) Clean them before re-using, you have no idea what's inside.

Steps one and 2 are pretty straight forward. But what about step 3? We get asked a lot on what the preferred method is. The answer will always be to flush them. But wait there's more! This is not just an article addressing the new A2L refrigerants and such, but more about prepping for compressor replacement due to burn-out, or replacing old equipment with new, and leaving the old line set, as it may be a more economical approach.  

If the "tech" before you didn't purge with nitrogen when brazing, or the compressor fried leaving acid through out the system, or if there is a different type of refrigerant oil being used on the new system (R22 to R410A or R22 to R32/R454B) we must make sure that the line set is free of any potential contaminants. A flushing agent does a great job of cleaning the line set, after all we are trying to achieve, CLEAN, DRY and TIGHT. However, flushing agents can sometimes be a pain to use or work with, and we certainly don't want to leave it in the line set either. Following it up with nitrogen is always recommended but, we can be sure that all of the flush has been removed from the lines. 

The best method I have ever seen to accomplish this with near 100% confidence is the Hilmor Lineset tool. I am the farthest thing from a salesman, and I usually butt heads with those in marketing. But I do know what works, and I do my best to advise you accordingly. Check out the video link and decide for yourself. 

NOTE: 2-3oz of flush in addition to the tool is recommended.