how to change send_charset on the fly?

[phyrster <phyrster@xxxxxxxxx>]

Hi mutters,

When I reply to some html format messages which have 'charset=gb2312' in
their headers, the send_charset became utf-8.

But the setting in my muttrc is:

|set send_charset="gb2312:GB2312:utf-8"

I am almost certain that my reply won't be readable for recipients who
uses gb2312 as the default encoding. 

My questions are: 

why my reply is not encoded according to the original
message's charset? 

How to change send_charset on the fly in mutt's 'send message page'?

-- 
regards
phyrster

You must be the change you wish to see in the world.
                -- Mahatma Gandhi