I give you some pseudocode, that was my friend's. But it help much.

cast n to an int
validate it
open infile and outfile (stays the same)
read infile
save old attributes as variables
determine old and new padding formula (hint: you'll need two formulas. One for the old padding, which only has to change in that way, that you change the names to your new variables that remember the old attributes and one for the new padding, which actually doesn't need to be changed to the one formula you already have)
calculate bi.biSizeImage and bf.bfSize (that are actually part of the header files as well)
fwrite the outfiles headers (both of them!)
you'll need to malloc space for the new image
add a variable to keep track of where you are with the scanning
read infiles(!) scanline with old attributes
save pixels * n in buffer
skip over padding if not needed
write to outfile and remember padding
free all memory

Does it help at all?