I was pondering the idea to lace it to a wider rim. Both rims are 29", but the ERD of the new rim is 6mm different from the original one, which results in shorter spokes. Is this an example of a bad idea?
I'd say that was less a bad idea than some, SJS did similar for me and I had no problems with it. Really bad idea is like changing a 26" to a 700c, that didn't work for me, but others have got away with it.
Every time you move the spokes is worse than not moving them, that goes for any hub, it isn't specifically a Rohloff thing. How much you move them, how often, how well matched the tensions, what use... everything will count, which is why it's impossible to quantify. For me, I wouldn't swap a rim unless it was absolutely necessary, I have one at the moment which isn't the ideal rim, but don't intend changing it. But then if I had to, I wouldn't worry too much about it.