Shell Programming and Scripting

Ok, so I have a shell script which runs a specific command, and that command sends it's output to the display. At certain times, the command sends buffered output, and at other times, the command sends unbuffered output in the form of a % progress bar, so if I run the command, the output I receive is similar to this:
$ commandname
Did some internal check...done (this line is buffered)
Did something else...done (this line is buffered)
Performing task: (this line is buffered)
0....10....20....30...etc (this line is unbuffered, and sends output to the display one character at a time)

I would like to somehow capture and reformat both the buffered and unbuffered output so that I can strip out all the command's output, and replace it with my own output. Without getting too specific, what commands can you recommend to do this? What I'd like to to is for each line of buffered output, replace it with different output, or no output at all, and for the unbuffered line, either change the formatting so that when the command sends a period, it prints something else. I realize I can do some pretty detailed stuff with sed, but am honestly not that experienced with it. Can anybody give some suggestions, guidance on how to go about this? To give an example of simple reformatting of the commands output, I could have all buffered output to be sent to /dev/null, and once the unbuffered output starts, send one line stating that all internal tests passed, then start sending the unbuffered output to the display again?

Sorry if this makes no sense, but any help is appreciated.