Ask HN: How to say “I continued working for free” in your resume?

I was holding down 2 part-time jobs at the same location, one of which was a sysadmin position.

Once a full-time sysadmin was found, I handed over that position but continued to work (unpaid) to assist in the transition and, later on, to help with high workloads and major outages.

In total, I did paid work for about 8 months, and unpaid work on the side for 2.5 years.

How do you put that down in your resume? Do you change the from-to dates to the full period of paid + unpaid work? Or do you separate the two? Add an N.B.?

I'm sure others have been in this position before. I'd be very grateful if you could share your experiences.