I remember going through the process twice:
The first time at the family home (about 1970 I think) when the teams took about three weeks to completely clear the area. The modified most modern devices in a matter of an hour or two. There had been an industry edict that appliances recently sold were fitted with either easily changed jets or even came with multi-gas burners. The longer tail of local operations were for older devices, mainly gas stoves, where various parts were taken out to a van with a bench and vice plus a selection of pipes, adaptors and tools to fit them. Sometimes, households were told that the old stuff couldn't be converted and (i think) were given a contribution to buying new.
Roll forward two years (I think), I live in Colchester and the switch was announced. Being young with a family, plenty of energy (compared with now) but little spare cash, I decided that I would fit central heating in the house myself. It was a thing of the '70s when DIY heating materials were easily available from many shops that sprung up. New houses were sold by builders without heating, or at condsiderably optional cost a rather poorly specified and fitted heating system. There was of course the rule that any gas installation work had to be inspected before use (nowhere near as strictly enforced as current legislation), so I decided to synchronise the full use of the boiler with the conversion programme. Thus when the surveyor came round to check on the devices, he took one look at the new boiler and ticked it off as being no problem. So the jet changes, gas valve setup and test was all done formally by the conversion team. Result!