Inconsistencies regarding nil coding-system

[Eli Zaretskii]

  (coding-system-change-eol-conversion nil 'unix) => nil
  (coding-system-change-text-conversion 'latin-1-dos nil) => undecided-dos
  (coding-system-base nil) => no-conversion

When primitives that encode/decode text accept nil as their
coding-system argument, they use `undecided' (AFAIK).  So the second
result above looks reasonable, but the first and the last are
inconsistent, and the last one is downright surprising.

Any reasons not to change coding-system-change-eol-conversion and
coding-system-base to consistently treat nil as `undecided'?  (If
agreed to, I suggest to make this change only on the trunk, not on the
release branch.)