In response, we have:

1. Archived the predecessors (currency reference year 2015, 1995 to 2018) by renaming with an "_15" specifier, marking as "discontinued", and segregating in the catalog.
2. Created one-for-one replacements (currency reference year 2016) with an unchanged start date.

The replacements reside in the historical catalog ...

• (Belgium » National Accounts » by Expenditure » Quarterly)
• (Belgium » National Accounts » by Value Add » Quarterly)

... and the predecessors under the associated "discontinued" nodes. They include, for example:

• NACEGDP_15CQ.IBEL   = [DISCONTINUED] National Accounts: by Expenditure - Gross domestic product, (Mil. Ch. 2015 EUR, CDASA)
• NACEGDPCQ.IBEL      = By expenditure - Gross domestic product, (Mil. Ch. 2016 EUR, CDASA)
• NALECICQ.IBEL       = By expenditure - Changes in inventories, (Mil. EUR, CDASA)
• NALSGVAIC_15UQ.IBEL = [DISCONTINUED] By value add - Gross value added - Information and communication, (Mil. EUR, NSA)
• NAJSDFGVAIDXCCQ.IBEL = By value add - Deflator - Gross value added - Industry excl. construction, (2016 Vol. Index=100, CDASA)

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