Those better systems would cost 10s perhaps even 100s of billions & unlike say a bank or other business, who can schedule IT cut overs for the weekend, the NHS has to be up and running 24/7.
Both old and new have run side by side, all bugs ironed out, seamlessly switched, then there is training on new systems, whilst fully operating the older ones.... which in most cases will not work on more modern operating systems.
All at the same time, as having a mtce team that is able to manage the new kit.
When i used to work in IT and went to a few hospitals, things like cat5/6 cabling, Full fibre internet, power, modern cloud based phone systems often didn't even exist, esp away from District Hospitals, often built long ago....
Our technicians became fewer and fewer as they retired/laid off, new ones not recruited, travelling huge distances, we'd use contractors, who would ring up tech support with "What do i do? not trained on this stuff/what does it look like?"
Its huge and why no Govt has done it... as recent events has shown, no one, even the wealthy, want to pay more tax.