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There's a mix, a quick search took me to https://londonbusadvertising.com/ which shows wraps, which aren't going to be replaced with a display. But also rectangle panels which could be replaced with displays.

Those panels might very well be vinyl for outdoor durability, but I don't see why they'd take 3 days to swap out, unless it's a scheduling/transport issue, for example a bus operator needs to drop off day before, so the ad company doesn't have to schedule around when the drop off happens, and the bus operator picks up the bus the day after, because they don't want to schedule around when the ad company finishes; now your one hour swap is a multi-day production.

A full wrap, could be a 3 day process though.