Is it okay to take a regional train in place of the designated long distance train on a Sparpreis ticket? For example, going from Heidelberg to Frankfurt, the routing might read something like this: NV*MA-Hbf 19:16 ICE376. I understand this to mean that you can take any local train (S-Bahn, RB, or RE) from Heidelberg to Mannheim, not just the connection that's printed on the ticket. From Mannheim to Frankfurt the ticket is restricted to ICE376 and you cannot take any other ICE or IC. However, would this ticket be valid on an RE from Mannheim to Frankfurt over Biblis, or an RB from Heidelberg to Frankfurt over Darmstadt? Just wondering if anyone has insight or experience on this; it seems like a "downgrade" might be allowed since (at least in this case) the local fare for the route is less than the Sparpreis ticket. But then again, maybe not?
The Sparpreis conditions say, "Nutzung von Zügen des Nahverkehrs ist im Vorlauf/Nachlauf möglich"; you can use local/regional trains to get to your ICE from Mannheim, on the same day, naturally, so the answer to your first question is likely "yes" if no specific local/regional train is stipulated on your ticket. But as for the RE to Frankfurt, the Sparpreis ticket is intended only for the IC, ICE, and EC trains (and CONNECTING regional trains) and valid only on the train named on your ticket, so you can't use it ONLY on regional trains - the Mannheim-Frankfurt train would not be a CONNECTING train in that case. You would need to refund the Sparpreis ticket in advance (15€) and purchase a new regional ticket. It's possible that a sympathetic train conductor would look the other way - who knows? - but such use of the Sparpreis ticket would not conform to official conditions of use.
You didn't hear this from me, but... they often don't even check tickets on the regional trains. Not that I'm recommending anything... Another option to get from Heidelberg Hbf to Mannheim Hbf is #5 Straßenbahn.
It seems to me I had a phone discussion with someone from DB about a similar question a few years ago. Someone wanted to go from Amsterdam to Koeln to Koblenz to Bacharach. They wanted the freedom to be in Koeln for an indeterminate time, then go to Koblenz. If they specified on the ticket an IC from Koeln to Koblenz, they would have to be on that train; they couldn't use the ticket for any other Bahn train (ICE/IC/EC) to Koblenz. However, if they booked it for a Nahverkehr (regional) train from Koeln to Koblenz (i.e. RE), they could stay in Koeln as long as they wanted, and take any regional train to Koblenz, as long as they got to Bacharach by 10:00 AM the following morning. They had to go directly to Koblenz if that was specified on the ticket, i.e., they could not take a side trip to Cochem. Two little known facts about Sparpreis tickets. 1) You can use the via function to specify stopover times in stations on the way. In 2008, I went from Cochem to Hannover with a two hour stopover in Koeln because I specified it at the time of booking. 2) You can manipulate the modes of transport to specifiy "all modes", "all except ICE", and "only local transport" for any leg, as long as at least one leg is on a Bahn train.