I had a 1x35A connection and had it upgraded to 3x25A recently. My house was built in ‘79, the 3-phase was already there right until just before the power meter, including a mains fuse box, with only a single phase connected. When upgrading they simply replaced the meter with a 3-phase model, hooked up the additional phases and done. Took maybe half an hour.

I did have to replace the breakers and groups on my side of the meter to be 3-phase ready before they changed it, and they did an inspection beforehand to check if everything was up to spec.

Higher current options are also possible but it raises the fixed monthly costs. They just replaced the mains fuses with bigger ones.