Very simply, upgrading an underground service is many thousands of dollars and if there is any possible need to do it then you need to stop right there and fully evaluate before moving forward.
Examples of the kind of disasters you want to avoid:
1) The underground service on a residence is 60 years old and only rated for 50A. The service panel is a piece of crap so you want to replace it but the minimum code requirement for a dwelling is 100A, so the inspector will not green-tag a new service panel unless the underground service conductors are upgraded.
2) An owner has added a 100A car charging circuit to a 125A service, without a permit. The main breaker is not tripping or anything, but if you show this panel an inspector there's a good chance that no final inspection will get approved unless the service is upgraded to 200A.
In either case, it's a disaster to put solar panels on the roof before you are sure either that the AHJ agrees not to require upgrading the underground service, or that the customer will pay for it.