There are a number of possible actions as a result, but they basically all end up with Germany getting hammered.
(1) CZ rolls over and plays dead in a couple of weeks. Very unlikely, really. They had a good army, and unless the country disintegrates politically, are going to give Germany a bloody nose. Germany will also lose the equipment it gathered, and probably lose the production (factories destroyed during the fighting or sabotaged). So at the best, in 1939 Germany is noticeably weaker. The only advantage for them is the earlier fighting probably has them operating more efficiently in Poland. Remember that at this point in time the best German tank is the Mk II.
(2) CZ fights for some time (a few months). The longer, the more damage is done to the German army and the LW. Also the greater the chance France at least will step in. Like option 1, but worse. Germany has a BIG problem in insufficient levels of shells, etc.
(3) France and or Russia honour their treaties. Germany takes even longer to beat CZ (they now have to leave considerable forces on the French border, even if the French do nothing). Depending on how well the CZ resistance goes, at some point France will have to do something. It's difficult for Russia to help directly due to the geography, but the longer things go on the greater chance a transit deal is arranged. Germany is now fighting 2/3 enemies.
(4) As 3, but Britain also decides to weigh in. This is a killer for Germany, as no matter what happens on the battlefield, they cant get any resources past the RN blockade, and now cant get them from Russia. Its very unlikely any of the Eastern European countries will help, they don't want to make themselves targets. Even if Russia is doing nothing much, the absence of Russian resources kills German production in a year or two. While a coup is probably a lot less likely than post-war accounts indicate, in this situation I could see it happening, the odds are simply too great. Having to keep forces in the East, for example, probably means an attack on France will fail.
Yes, their enemies are far less prepared in 1938, but so is Germany. In fact, Germany got relatively stronger in 1938-9 due to its spending, this wont now happen.