What few people understand about software, is that software is like Swiss cheese. Full of holes, but only the ones that matter get found and fixed , by testing and through use. There will always be a few that matter and only show up later though.
I worked for many years on a voice switch very similar to the one at Swanwick, and software upgrades in the field were always worrying. The new software would first be tested at the manufacturers facility, then we (ops and engineering) would the run our own testing scenarios at our technical centre for a couple of weeks, then we would head off to one of the smaller centres and install the upgrade on a night shift. We would "split" the system and upgrade one side of the switch with the new code, run that for an hour or so and if that worked we'd switch sides and then upgrade the other switch. Next the two systems were put back into full redundancy mode and hopefully would play nicely together. All this would take upwards of 8 hours, and the system HAD to be fully operational by 0600. Even once the system was bak, the next few hours were still critical as controllers had the ability to dig up anomalies that we never could find in testing.
Many 16 hour shifts were spent doing this .... I have the grey hair to prove it!