The proper way to change time signature in the middle of a bar?

[Wols Lists]

I've got a piece of music where I'm doing the following ..

\time 2/4 c2 | c4.
\time 6/8 \partial 8 c8 | c2.

This gives me the warning

voiceStaff.ily:38:2: warning: trying to use \partial after the start of
a piece

but it works fine. Without the \partial, though, lilypond gets confused
over the length of the measure (no wonder :-), so what's the proper way
of dealing with two part bars in different time signatures running into
each other?

Cheers,
Wol