The Lord requires a cleansing of impurities and it was also a step of faith for Gideon to tear down the altar and the pole.
The positive "replacement", if you will, is that which allowed Gideon to build an altar to the Lord, offer a sacrifice and worship Him...something that hadn't been done openly for quite some time it appears. A precious opportunity for Gideon to make a speech without saying a word, huh?
Gideon is risking not only the already hostile community's rath but also that of his family. After all it appears that his father is the "keeper" of the altar as it is built on his land. Daytime, nighttime ~ does it really have to matter as far as his faith is concerned? The fact that he made the decision to obey and picked the time when he most likely could get it done without a fight from the locals or family...well, I say go for it.