replace text: Not knowing the line number

[Nagrik]

Hello Group,

I have a file named myfile

it has a line starting with

#% Some text

I don't know the line number of this line and at one time I want to
replace it to

% Some text

and later back to

#% Some text

Can somebody please tell me a way to do it in sed. I am a beginner
for sed. I will greatly appreciate this help.

Thank you very much.

nagrik.