Switching would be a pretty significant effort. Implementing one of the commercial network monitoring products is a lot of work (which is one of the things they're typically criticized for). Implementing Nagios is also a fair amount of work. Replacing a commercial product once you've got it up and running is quite a bit of work; for that reason, most Nagios implementations are greenfield situations. In terms of tips, one advantage of having a commercial product in and working is that it will give you detailed stats of what machines, apps, etc., you're monitoring, which would give you a significant leg up in terms of migration planning.
This was first published in May 2005