There is no "sliding scale" on the date conversion. IOW, there is no
intention to adjust this as the years progress.
And it's pretty limited, even if you are going from 8 digit to 7 digit.
From the help.
Only valid dates can be converted. If either the from-format or the
to-format use only 2 digits to specify the year (for example, *MDY,
*DMY, *YMD, or *JUL), valid dates are in the range of January 1,
1940, to December 31, 2039. Otherwise, valid dates are in the range
of August 24, 1928, to May 9, 2071. If the year is specified with
only 2 digits, years in the range of 40 to 99 are assumed to be 1940
to 1999; years in the range 00 to 39 are assumed to be 2000 to 2039.
The command works in conjunction with the QLEAPADJ system value.
Now why it can only convert *YYMD from 20710509 into *CYMD 1710509 and not
*YYMD 20710510 into *CYMD 1710510 escapes me.
I think I would avoid this crap and start using 'real' date columns.