The German Constitution can be changed by the German Parliament (which is why it could be argued that the independence of the Bundesbank from political interference and pressure is slightly mythical), but Karlsruhe has essentially stated in its Lisbon Treaty Judgment that at some point a referendum would be needed. So despite the outcry from the governing parties that the SPD are being irresponsible in backing Eurobonds:
"Merkel spokesman Georg Streiter said the a German referendum "lies a very long way in the future"..."
Any fiscal union will need explicit consent from the people of the Eurozone, and there has to be an open political debate about the alternatives with competing proposals. The step-by-step approach that has been taken so far (in Europe generally, but in Germany and by Merkel especially) has a corrosive effect on the confidence in national and European political leadership and ability, and in the idea that there is a solution. What we have now is a strategy that breeds cynicism, to the extent that it's hard to know if there is a strategy at all and we have to engage with a new type of Kremlinology centred entirely on the contents of Merkel's head! Without even a debate on the future of fiscal union, it's hard to see any political deal produced at the end of this process being accepted after all the suspicion and bickering that will likely continue for another 2 years, if not longer.
So the debate is necessary for any plan to have a hope of working. Speaking in favour of Eurobonds is not the same as making an open commitment to Eurobonds to be introduced as soon as possible (the SPD are still quite close to the government on conditionality, but with more solidarity); it needs to be part of a deal that covers conditions and democratic oversight. It cannot be a technical fix introduced at breaking point, but the product of an open political process. Utopian to hope for given all the summitry, but necessary if we're to have any hope of creating a workable compromise.
Note: Juergen Habermas' (et al) article in the Frankfurter Allgemeine Zeitung (here in German) has been linked to the SPD's move, and Habermas will be involved in the SPD's manifesto for the next German elections. Other interesting articles on the SPD's website on Eurobonds and meeting their Spanish counterparts here and here.