Why the desire to change? I know of no breakers better than the Zinsco that will fit. Challenger makes breakers that will fit the Zinsco and they are terribly expensive and appear to be identical in every respect. Zinsco is my least favorite but my customers have not had problems with them for several years now. Your practical choices are to leave it alone or replace absolutely all of it. You know the cost. Just decide.

Well, I had tested one of the 20A breakers at random, by loading it with 30A for 10 seconds and it did NOT trip. In retrospect, I realize that I was perhaps too optimistic in expecting it to respond to overcurrent so quickly after seeing some typical breaker trip curves online.

But experienced posters on forums seem to have indicated that Zinsco main panels have other problems beyond the breakers, eg. with the busses fusing to the breakers, through mine seem fine.

A breaker fusing to the buss would not of itself cause problems. Zinsco uses aluminum buss and the supposed fusing is really oxidation. Aluminum oxide is poor conductor. Anti-oxidation compound would be good for such buss bars. New breakers will not cure not halt oxidation.
Tell us about the other forum. I should like to see the other comments, might learn something.

There are a lot of older homes with Zinsco and Federal Pacific boxes that have operated for decades without any problems. However, it looks like your service may be 100 amps. If you think the present panel is meeting the power requirements of your home, you might consider leaving it alone. My guess is your home is at least 30 years old.

Some important considerations are that recepticles and switches are grounded and that kitchen counter top and bathrooms are GFCI protected.

Your present system is grand fathered. BUT, replace and upgrade your service and suddenly you are led into the 21 century and the latest NEC requirements. In this regard, $2,100 is a bit of a bargain.

"Your present system is grand fathered. BUT, replace and upgrade your service and suddenly you are led into the 21 century and the latest NEC requirements."

This.

The system grounding will have to be brought up to date for the code you are under now.

That likely till require a new grounding electrode, and possibly more.

many AHJs use a service change as an opportunity to pole at everything in the house.

They seem to fail to understand that by driving up cost they reduce any upgrades.

That is exactly my point. Opening my family up to be forcibly brought to the 21st century (well, I am, my house is not) would be akin to let the camel's nose under the tent.

If any authority gets involved, there are enough loose ends in a mid 50's house to be de-grandfathered and act as an excuse for the city to try to balance its entire retirement fund on my main panel change.

Many circuits in my house are not grounded (there simply isn't a ground wire). I have put GFCIs on the first receptacle of every circuit, so in principle, all my outlets are somewhat protected. Also, every time I open a wall or do some significant work, I run a ground wire. But I'd rather have that happen on my schedule and budget rather than immediate AHJ decree, which would also preclude any DIY. So while electricians may give me an $2200 estimate, sure enough, other 1950 vs. 2012 code problems will most likely be encountered along the way, and I'd soon find myself begging the city (my "community", "the people") for temporary exemptions to keep my house designated as habitable while I'm given 6 months to upgrade my entire electrical circuit' if the upgrades stay limited to electricity.

So given the fact that I am an electrical engineer, I should be able to find some reasonable solution to this, with some guidance and... if "the people" will only allow me. Otherwise, as I know from history, when the crowd disagrees, you just drink hemlock like Socrates, or eventually get crucified. Having migrated from collectivist Europe to a (once) freer America I'm very familiar and sensitive to those issues, and watch America's Europeanization (and inevitably same decline) with great sadness. But that's another story...

Previous discussion on the same topic can be found at:

http://www.doityourself.com/forum/electrical-c-d-c/465872-main-panel-rework-pictures.html

All you can do is ask the AHJ what other requirements they will levy if you upgrade the panel.

Some are downright stupid about trying to get everything up to the most recent code (despite the old work being grandfathered, something some AHJs seem to greatly resent).

Grounding electrodes are not that big a deal, but should be upgraded with a new panel.The old electrodes and connections may not be in the best of shape.

The use of a GFCI receptacle to protect downstream receptacles when no ground wire is available is code compliant.

Simply adding a ground wire may not meet with the AHJs approval.
It needs to be run with the circuit conductors, and some AHJs interpret that to be in side the same cable, as opposed to on the outside of an old 2 conductor cable.