I am looking to purchase a home and it was built in 1957. It has 100 amp service what would be involved in switching it over to 200 amp service

If it is fed overhead by the utility, then the conduit riser and weather head, meter, conduit between meter and panel, all new larger wire in all conduit, or cable, panelboard and breakers, and larger grounding cable, and ground rods.

Depending on location and labor market, and types of materials installed, cost can run $1000.00 to $2000.00. Most likely need a permit and consultation with the utility company. A well organized contractor should handle all these tasks. Plus he should have the proper licensing and liability and worker comp insurances.