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It's more a job for perl whose regexp can more easily find matching pairs. Anyway, the few sed implementations that have a -i option for in-place editing have copied it from perl and in ways incompatible between each other.

If we assume that {/} are always matched even inside "..." strings, that could be

perl -0777 -i -pe '
  s{
    ^ \h* filter \s* \{
      ( (?: [^{}]++ | \{ (?1) \} ) * )
    \}
  }{
    $& =~ s{ ^ \h* \K \# \h* (year_n \h* =) }{$1}gmrx
  }gmxe' -- your-file

Where:

• -p enables the sed mode where files are processed one record at a time where the equivalent of sed's pattern space is the $_ variable (on which s{pattern}{replacement}flags operates by default), like sed's s.
• -i for in-place editing (copied by some sed implementations).
• -0777 changes the record separator from the default of newline (like in sed) to some impossible byte value, so the files are processed as a whole. Same as -g in newer versions of perl.
• then we have a s{pattern}{replacement}gmxe where:
  • x allows adding whitespace (and comments) in the pattern to improve legibility.
  • m makes it so that ^ matches at the start of every line in the subject instead of just as the start of the subject.
  • e is for the replacement to be interpreted as perl code.
  • \s is for any whitespace (well ASCII only by default) including newline, similar to [[:space:]] in POSIX regexp, and \h for horizontal whitespace (ISO8859-1 ones by default, that is space, tab, and non-breaking-space encoded as 0xA0, but importantly not newline; similar to POSIX' [[:blank:]]).
  • The important part in there is (?1) which recalls the regexp in the first (...) capture group so allows for recursive regexps.
  • The replacement applies another s{pattern}{replacement}gmrx to $& which is what was matched by the first regexp with:
    • r returns the result of the substitution instead of applying it in place to $&
    • \K marks the start of what's to Keep from the match, so we don't discard what's matched to the left of it.

To find those files that have such matches so only those be edited, you can do something:

export REGEX='
  ^ \h* filter \s* \{
    ( (?: [^{}]++ | \{ (?1) \} ) * )
  \}'

find -H config_dir -type f -exec perl -0777 -ne '
  print "$ARGV\0" if m{$ENV{REGEX}}mx' {} + |
  xargs -r0 perl -0777 -i -pe '
  s{$ENV{REGEX}}{
    $& =~ s{ ^ \h* \K \# \h* (year_n \h* =) }{$1}gmrx
  }gmxe' --