So many pubs say, "don't swap bolt assemblies between rifles," or else, or else, or else!

Well, why not? Does a "worn down" bolt fail to properly lock with a new barrel? Does this affect anything more than accuracy?

And is there anything wrong with using a new bolt in an older barrel?

Thanks.

if the bolt is not head spaced correctly the gun blows up

btt

Quoted: Does this affect anything more than accuracy?

Changing bolts can increase the wear on the bolt and locking lugs - until they 'mate up' - then if you switch again the 'mating' process begins anew..

Yes if the bolt is worn far enough headspace could increase to dangerouse levels, but it would have to be a LARGE amount of wear (see Hatcher's Notebook for more information on this subject).

And is there anything wrong with using a new bolt in an older barrel?

Nothing - in fact it's the first 'fix' you are supposed to attempt when your current bolt + barrel fail the field gauge test.