Is it worth upgrading my old Ender 3 or just replace it?

I pulled my old Ender 3 out of storage about a year back in almost the same state, so here's what actually mattered. First thing I did was replace the belts, the PTFE tube, and the nozzle, all cheap stuff, because anything rubber or plastic degrades sitting in a garage. Then I checked the power supply connections and the board for any corrosion before plugging it in. Once it printed a decent test cube I decided whether to invest further, and honestly the print quality was fine, just slow.

For firmware, the stock 8-bit board can still run Marlin with bed leveling support, but it's cramped. The common path people take is a cheap mainboard swap with the newer 32-bit chips, which opens up proper firmware and smoother printing. If you'd rather not fiddle, a bed leveling probe on the stock board plus a reflash gets you 90 percent of the benefit for long solo prints where you can't babysit the first layer. Direct drive for cosplay parts? I'd skip it. You're printing PLA or PETG, not flexibles, and a good bowden setup keeps the hotend light which matters on the stock motion system. Spend that money on a better cooling duct and maybe yellow springs for the bed instead.

Honestly though, weigh what your time is worth. Long prints mean you want reliability, and a machine that sat for years will eat a weekend here and there. If the revival goes smoothly and you enjoy tinkering, keep at it. If the first few prints turn into a repair saga, the money you'd sink into mods gets you most of the way to a modern printer that does auto leveling out of the box. For sturdy props, PETG is the obvious step up from PLA since it's way less brittle and handles being dropped, though it strings a bit more and likes a dry box. If you want parts that survive sanding and filing without snapping, that's the one I'd go with. Keep the old spools for painting tests, garage air and humidity have almost certainly made them brittle by now.

Gotta push back on the PETG advice for props. Everyone repeats that it's tougher, but it's also a nightmare to sand and it fuzzes up when you file it, which is half of cosplay finishing. I switched most of my armor pieces to a tough PLA formula a while back and never looked back. It still takes a knock way better than basic PLA but it sands and primes like the stuff you alredy know. PETG also loves to string on long pieces and you'll be picking whiskers off every edge. If the piece is pure size and getting painted anyway, tough PLA is honestly the easier road.

One thing I'd check before buying anything is the little plastic lever arm on the stock extruder, mine had a hairline crack from sitting under tension and it made feeding inconsistent for weeks before I spotted it. Also worth looking at whether your bed surface has gone dead flat, becasue a garage that swings between hot and cold can warp the plate enough that a leveling probe just maps the warp instead of fixing it. On the firmware question, it depends which board revision you actualy have, there were a few over the years and they take diferent builds, so pull the cover and read the silkscreen before you order anything.