I have an old IBM thinkpad Z61t, and I want to upgrade the screen to one from an even older Toshiba laptop. The screens are both very different sizes, and probably different resolutions, but the same aspect ratio (16:9). The connectors are all the same and it plugs in just fine, but I haven't booted it up yet because I don't know if its a good idea. I don't know whether or not it'll work, and I don't want to risk wrecking the screen. I do know a bit about hardware - I've completely dissected a number of laptops and scavenged parts to build one that's better than any of the original ones were, so I do know what I'm doing. I'm just wondering if it'll work, and, if not, what the worst that can happen would be.
It should work - and if it doesn't, the screen just won't turn on.
Computers query the resolutions of monitors to negotiate the best possible resolution supported by the monitor (even for VGA connections - using EDID). EDID predates your laptop.
Unless you specifically override the negotiation, the screen can't be damaged, and even then, its most likely a modern screen will handle it - either by not turning on or by scaling the content.