Alongside the Classic Stampin' Pad redesign debut in the 2018-2019 Annual Catalogue, Stampin' Up! improve the ink as well! You may have noticed that the old ink would sometimes bubble (especially on new stamp pads and with lighter colours), which can give uneven stamping. So, Stampin' Up! have added a defoamer to the ink ingredients. This will reduce ink bubbles, giving you an even coating of ink on a stamp, resulting in a smoother, solid stamped image.
The colour formula for the ink itself has not changed, all that's been added is the defoamer. That means that ink refills purchased after 1 June 2018 can be used to re-ink stamp pads purchased before 1 June 2018, and you can get the added benefit of the defoaming agent.
Also, ink refills purchased before 1 June 2018 can be used to re-ink the new Classic Stampin' Pads. However, you need to be aware that the those ink refills did not contain the defoaming agent.
This only applies to Classic Stampin' Pads and Classic Stampin' Ink Refills. Do not use the Archival Stampin' Ink Refills to re-ink the current Classic Stampin' Pads. The Archival Stampin' Ink Refills were available before 1 June 2018 in Basic Black and Basic Gray only.