I have a gas water heater (40 gallon tank). I want to change it to an electric tankless water heater. I have read that I need 200 amps in the house to do this, but currently I have only 100 amps. The outside meter is located only about 25 feet away, but the lines are underground. What is involved with upgrading to 200 amps, what would be a ballpark cost for an electrician to do this, and does being underground really increase the costs to upgrade? I really dislike gas and this is the reason I would like to change to all electric. Thank you.

Then why no just install a regular tank type electric heater?

These tankless heaters are NOT all they are hyped up to be.

YES, underground make a service replacement much more expensive. Although 25' is not that much.
A typical upgrade is in the $1800-$3500 range depending on your location, with the high end in the California and west coast areas. ADD to that the extra cost for the underground portion.