Sherparsa, 
I know exactly what you mean about the clicky keyboards! I have an old-skool IBM "Type M" keyboard and it's as you describe. This and the non-clicky Sun Type 5ish keyboards are my favourites. Being a geeky type, I looked into it and the clicky type of keyboards are called "buckling spring" keyboards. On my Type M, you can't interchange *all* the keys, because they have different sloping edges on some keys (particularly arrow keys). It might be possible but look odd, I don't remember. I am pretty sure you could get e.g. a Dvorak layout, though.

If you can't find a second-hand Type M in good nick, these guys sold a newly built (but still buckling spring) keyboard to a good mate of mine:
http://pckeyboards.stores.yahoo.net/
The one he bought feels exactly like my original IBM, though it also has a touchpad and trackpoint. (Although my IBM is ~16-20 years old, the action is still nearly 100% sharp.)
They also sell these keyboards with touchpoints (nipples), touchpads, all sorts of extra gubbins, if you want them. I believe they also offer custom layouts. (NB: I am not involved with this company in any way, they're just the /only/ seller I could find for new buckling spring keyboards.
If you want a second-hand Type M:
http://www.clickykeyboards.com/ (and eBay or whoever)

The non buckling spring keyboard I use at work (they won't pay £100 for a kb for me!) is a KeyTronic KT2001, which I highly recommend if you're on a budget.

How much of a saddo do I feel having written all that? ;¬) I hope it helps!

Nice article here about the difference between QWERTY and Dvorak keyboards. 

Have been training myself on the Dvorak too for sometime so I'd master it as I have the QWERTY.

You have talked about how to scratch off keyboard markings and then repaint them with new keyboard layouts using a pen of some kind (and i'm sure you have been joking mostly otherwise a small sticker addition to the present keyboard can also take care of the job without necessarily ruining the old layout that may come in handy some time anyway!) ;-)

There are ways that a keyboard's hardware layout can be changed easily, yes, but not sure with today keyboards though!

Back in the old days, (as old as a decade or so 'old' that is) there were certain professional keyboards with each key equipped with a 'cover' upon which the character had been printed. The 'cover' was exactly the same shape as the 'blank / no prints' key underneath it, only slightly larger to snap-fit in. If you changed the keymap layout to whatever you liked and for whatever the reason, then you could just remove that top cover thing form the key(s) underneath it by pulling it out, and then replace them with the top cover thing of the other key(s), just as simple as that.

Anyone knows how to find such keyboards today? do they come in 'ergo' design shape too? are there any other methods of having the physical keyboard hardware layout changed even easier than that?

You bet!

Sherparsa