Make it into a matrix. If there is at least one character column in your data frame, it'll coerce the rest to character to match, since you can only have on type of data in a matrix.
new <- as.matrix(old_data_frame)
If there are no character columns in your old data frame, do:
new <- matrix(as.character(as.numeric(as.matrix(old_data_frame))),
ncol=ncol(old_data_frame))