Arnold, your question is a good one and deserves a serious reply. Here's an attempt.
I think it is ok to make major changes in basses under certain circumstances. If I own the bass and I know that something's not right with it, it doesn't bother me a bit to regraduate or rebar the instrument, change the overstand etc. If it belongs to a customer, I still think it's ok as long as the customer is in agreement. I wouldn't make any changes without the customers consent.
Before I got into this business, I bought an old bass that had the names of every one who had worked on it written on the inside of the top going back as far as the 1850s. When I got it repaired the repairman added almost as an aside that he "cleaned the top off for me". In doing so he erased all of those names, places and dates. I'm still mad about it, and it's one of the reasons I became interested in doing bass repair myself. That's an example of an improper and irresponsible act. Now, if he had had to do it to make the bass playable, I might have a differrent view.
What I'm saying applies to 95 to 99% of the basses that come through the shop. I would spend a lot more thought on a really valuable old instrument before making or suggesting changes. It would be irresponsible to regraduate a Panormo bass without absolutely considering everthing else first. But what if you found that the Panormo had ribs 1 or 2 mm thicker than what he normally did? Hmmm?
A few years ago I rebared a very famous jazz musician's bass, but I showed the old bar to the present owner first and explained what was wrong with it. It had been rebared in the early 50s with what I can only describe as a bass bar that bore more resemblance to a 2x4 than a proper bass bar, and that work had been done after the "famous" player owned it. I believe I restored it to much more like the original than what was in there at the time. Another example, so that people don't think I'm talking about just making some changes with a chain saw: I've owned two old three string basses that I changed to four strings. I don't think anyone would have a problem with that, and of course it could be changed back. I've regraduated many modern German commercial basses that were just way too thick to produce a good sound. I'm not talking about taking a bass with good thicknesses and reducing them willy-nilly. Many commercial German violins, violas and cellos have the same problems. An article in one of the national magazines stated recently that the Germans started leaving the instrument deliberately too thick in the hopes of avoiding cracking while the instruments were in transit. This was started in the 30s.
Here's another example: many old basses have tuning grears that just don't work. They slip, they're noisey, they rattle. If the gears have histroic value it is sometimes possible to have a new part made in the fashion of the old.
A bass is a tool for a musician to use. All old basses have been modified to some degree for modern use. Several years ago I had an old Bohemian bass with a 44 1/2 string length! I did not feel guilty about cutting it down to 41 3/4.
Perhaps the most famous change to a bass may be the cello shaped Bergonzi that Vuillume lengthened. It was essentially unplayable for modern playing styles. Hundreds of copies of the
bass have been made in Mirecourt and by other French makers immitating Vuillumes changes. There's a picture of this bass in Duane Rosengaards book CREMONESE DOUBLE BASSES.