Migration from SQL Server and Problems in applications and Database connection URL of application

I am working on the project where we are migrating from SQL Server 2000 to 2005 and also migrating from 2005 to 2012 or 2014. There are 30 databases that supports more than 45 applications. Some of the applications are Windows desktop and some are web based application.

My question is if I change the server than there will be problem in connectionUrl and I have to change the connectionURL of all the applications in the code, right?

Is there any way that I do not have to change the connectionURL in the code of all the application.?

What do you prefer?

PS: Most of the application are in C#, ASP.net