I once worked at a place that transitioned from formal PTO to "unlimited" PTO. Many of us had accrued a good chunk of time on the old system, and the rules of the old system was that it carried year after year. When "unlimited" PTO was implemented, management decided not to reimburse us for our accrued time, since "it was all now covered by 'unlimited'" and so we didn't technically "lose" any time. Didn't feel right, but I didn't know how to voice it at the time.