There's a few angles to this.
The length & extent of outage required when upgrade works are undertaken depends on the nature of the tech or carrier-add, and individual site config. There is not a one-size-fits-all solution, but wherever existing techs can be left on-air or restored sooner than new ones are brought live, they will be. It's just not always possible. If a cherrypicker is being used, that suggests that some alterations or equipment swaps are being carried out at height.
In the same way, the coverage footprint of each site differs. There are multiple sites serving the town, and quite often in urban areas there is some degree of overlap between sites, such that an outage on one (planned or unplanned) won't necessarily cause any loss-of-service. But it varies, and sometimes some loss-of-service is unavoidable. It's certainly not done with any vindictive intent.
Whilst the core network has some similarities with an ISP, the radio network is somewhat different.
