I agree with everyone that says it can be done. But on such a small system......
On a residential system that only holds 8lbs I would put new, clean refrigerant in it to absolutely know for sure its clean and pure. The extra time I would have to take to filter out and pull a good vac on the recovery tank (to be sure I got a good one from the supply house) and filter in for only 8 lbs does not make it worth it. A commercial system that hold 40lbs is a different story.
Filter driers dont filter N2 or O2 and there are too many RNC installers that run a vac pump for 10 minutes until the "tone change" and call it good. If I originally put the system in and know it had a <500 micron vac pulled, then I would re-use.
FWIW: i would not put a HRU on any system that still has a warranty. (My cummins diesel truck did not get any additional performance enhancements till the warranty was up.)
Give a man a fish, he will eat for a day. Teach a man to fish, he will eat for a lifetime.
Give a man a capacitor, doesn't know what to do. Teach a man to install it, now he knows everything.