The source explains (citation (pdf)):

KOF updates the set of underlying indicators and their weights annually after the release of previous year’s annual Gross Domestic Product (GDP) data by the Swiss Federal Statistical Office (FSO). The 2019 revision took place in September.

Our reference series is the smoothed continuous growth rate of Swiss GDP distributed across the three months of a quarter, based on the official quarterly real GDP statistics, adjusted for the effects of major international sporting events, released by the Swiss State Secretariat for Economic Affairs (SECO) in early September 2019. SECO in turn takes the release of the previous year’s annual GDP data published by the FSO into account.

The 2018 vintage of the KOF Barometer (published until August 2019) comprised 345 indicator variables. The current 2019 vintage, which is now replacing the 2018 vintage, consists of 376 indicator variables, selected from a pool of more than 600 candidate variables. Compared to the previous vintage, 75 indicators are new and 45 dropped out of the set of selected indicators

In response, we have:

1. Archived the "2018 vintage" series (2008 to 2017=100, 1991m1 to 2019m8) by renaming with an "_17" specifier, and by marking its metadata and catalog entry as "discontinued."
2. Created the replacement (2009 to 2018=100), which has an unchanged start date.

The series reside in the historical catalog (Switzerland » Cyclical Indicators » KOF Leading Indicator) and are:

• CIIEACTF_17UM.ICHE = [DISCONTINUED] KOF leading indicator - Economic barometer, (Index 2008 to 2017=100, NSA)
• CIIEACTFUM.ICHE    = KOF leading indicator - Economic barometer, (Index 2009 to 2018=100, NSA)

Because catalog locations are subject to change, the upper-right search box on DataBuffet.com provides a "find in catalog" mode that accepts a mnemonic.

References

• KOF Economic Barometer: Downward dynamic continues » KOF Economic Barometer and reference series: annual update (Sep 2019)
• Baro change 2019 background information (pdf)