Woooo interesting questions.
Mostly because I get to argue the counterpoint on pretty much all of them for you 😉

Replacement miniatures: valid options or just play with the original stuff?
Absolutely. In fact, many games don’t just look better, but play better with replacement pieces.
I don’t think I’ve played Blood Bowl or Space Hulk with original minis for about twenty years 🙂

Rebasing your miniatures for a new game: Would you do it?
Absolutely. In fact, I just have. All my cyberpunk minis – I was really unhappy with the basing. Then I followed the example from MiniatureHobbyist and laser-cut myself a load of 2mm clear acrylic discs. They look superb on thin, clear, bases – and they work for any game on any terrain, in any setting.

Re-using old stuff: what the oldes part of your collection that still sees action in gaming?
My Space Hulk tiles occasionally get a run out (though I prefer using scratch-built terrain and the occasional 3d printed corridor!) They must be 30 years old by now. I still use my original yelllow-with-black numbered D8 for Blood Bowl that I bought sometime around 1989-1990 (2nd edition with Astrogranite pitch – sadly lost to the mists of time).

I’ve been back on the reminiscing train this week. Here’s a totally magnificent tune:

Pledges – following in @sundancer ‘s steps, I’ve taken a few days off work; so I started painting up the Terminators I magnetised a few weeks back. I’ve only got the zenith base done and slopped on some skeleton horde contrast paint, but they’re looking pretty impressive already! So this week’s pledge is to get them finished (with photographic evidence posted here) before the next forum thread….