When I was a kid (during the 80s classic space era), part changes were a rare, rare thing for Lego to do, and when they made a change to a part, it was a change that made sense. Case in point would be the "lamp holder":
This thing was constantly in danger of twisting off, especially in the space sets that called for an antenna piece to be twisted into the hole. I can't count how many of these things I broke (although at least I was left with a serviceable 1x1 stud when I was done). Eventually, Lego got wise and retooled the part to be stronger:
This change clearly necessary. In fact, I'd always wondered why Lego hadn't just designed the part that way in the first place.
During the entire first decade of my Lego collecting experience, I can only remember there being two or three significant changes to staple elements, all of which were intuitive. First came the thin-chinstrap classic space helmets, then thick chinstraps, then Futuron helmets. Those old straps were always snapping, so replacing them with a stronger design was totally reasonable. Then came hollow-stud minifig heads, which make sense from a part tolerance standpoint, since the air inside the head needs to escape when being placed on the neck peg.
From about 1978-98, Lego was always adding new parts, but rarely phasing out or replacing old ones. There was a lot of room for the element catalog to grow back then. But in the last four to five years, it seems like pieces have been getting phased-out or retooled at a breakneck pace. As if messing with the greys and browns wasn't enough, Lego phased out what was, in my opinion, one of their most user-friendly canopy styles:
I understand why they phased out the old hinge plates, since they could be a bit loose at times. But it was so much easier to design with those hinged canopies, and I always felt that phasing those out was a step backward design-wise.
Then they phased out those nice, modular wing pieces:
These were much easier to lock together and make larger wings out of. They'd been in the Lego stable for more than twenty years, then discontinued. I guess they weren't too compliant with the standards of Star Wars sets.
Then, just three or four years ago, Lego had to meddle with these:
It's much harder to design Galaxy Explorer-type ships with those awkward thin side-walls, since they don't line up with the clear bricks. Another perfectly good part that was fine for nearly 30 years, then changed for no apparent reason. Unless they were just cheaper to mold since they used a fraction less plastic
Just two years ago, this one cropped up:
Was peg slippage a major problem or something? I seem to remember the non-groove version of this mini cone working just fine, not a single complaint about it in its 23 year lifespan. I guess this change isn't a total game-ender, just the most superfluous in my opinion.
I've always been a fan of those old slanted space windows.
Oops, never mind.
This one worried me when I first saw it:
I thought that maybe it was replacing this:
I really hope not, since I rather enjoy being able to set recessed tiles into brick walls. Not to mention the fact that the "headlight brick" has that useful square hole in the back for perpendicular connections. Hopefully I'm just being paranoid about this one.
Last but not least we come to one of the most irking, and most recent, changes. I'm talking of course about the classic raygun:
It was recently replaced with this rather blunt-looking instrument:
At first I held out hope that maybe this was simply a new variant. But when I opened up my second Smash N Grab set, I found that the ribbed rayguns included with the first runs of Smash N Grab had been replaced by the new versions. So I guess it's official, Lego has nixed one of its most iconic accessories. The reason I find this particular change so aggravating is because the old school raygun design is to Classic Space what the lightsaber is to Star Wars. For 30+ years, this had been the trusted blaster of every space traveler from Classic Space to (early) Space Police III.
I wonder what's going through the minds of the Lego Part Tooling Department when they make changes like these? Are they honestly responding to real issues or complaints? Or have they simply run out of new parts to design, so now they're fiddling around with the old part catalog so that they can justify the continued existence of their jobs? It's one thing when Lego makes a barely noticeable change to a piece, like the little ridges inside the basic bricks, serial number changes, etc. Those are not significantly noticeable. But when you drastically alter something like the classic raygun, you're doing more than simply "updating" an old accessory, you're meddling with our childhood memories. The one thing I've always enjoyed about Lego is that I could go to a store, buy a set that was released two weeks ago, and find brand new shiny versions of old elements that had been around for 20 years or more. It may sound silly, but I like the fact that certain old pieces are still "alive" and in circulation. But over the last five to six years, I've seen more and more of the Classic Space universe fading from existence, bit by bit, mold by mold.
Note to TLG: If it ain't broke, don't fix it. Enough's enough!
I like those newer bricks too, and it's nice not to have to use a technic 1x1 brick with a blue peg-pin in it whenever I want a 1x1 perpendicular connection. 
