I'd say technically speaking, you're already illegal there, unless the law covers you while a renewal is being processed (which is the case in many countries with residency permits). That would then go to whether or not you received the receipt for request before the expiration date, and local law.
In practicality, you're traveling within the borderless zone, and unless you do something stupid, have very, very little risk of being questioned over the legality of your presence. Outside of special circumstances (checking the news might be one way to do this), checking documents at borders within the Schengen area is prohibited by EU law. So, again, unless you run afoul of the law for some other reason, you shouldn't be asked for your documents barring extreme circumstances (I recently traveled to Switzerland from France, and read about additional checks because of the riots in France, but in reality there were no checks, and the customs office wasn't even open for basic declarations, as Switzerland is in Schengen, but outside EU - so sometimes even standard, basic checks aren't carried out for whatever reason within Schengen).
Point is, I wouldn't worry.