In a direct communication, the source explained:

 In the end of 2017 was administrative-territorial reform and time series were broken.

We have plan to calculate same years backwards in all tables with "U" (tables by new administrative division), to create new time series.

In response, we have:

1. Archived the predecessors (January 1989 to December 2016) by renaming with an "_16" specifier, by marking metadata as "discontinued," and by segregating in the catalog.
2. Created one-for-one replacements. They start at January 2017. The corpus and properties are otherwise unchanged.

The series reside in the historical catalog (Estonia » Demographics » Vital statistics » Monthly » By date of occurrence) and include, for example:

• VSQDTH_16UM.IEST    = [DISCONTINUED] Vital statistics: Deaths, (#, NSA)
• VSQDTHUM.IEST       = Vital statistics: Deaths, (#, NSA)
• VSQDTHUM.IEST_0044  = Deaths - for LAU 1 Ida-Viru maakond
• VSQDTHUM.IEST_00784 = Deaths - for Tallinn

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.