Some of the changes are because .Net is not just the next version of VB but a new runtime that is common to all .Net languages, some compromises in naming convention have had to be made regardless of previous language experience. Hence why text boxes are now TextBox rather than just Text. Listboxes on the other hand behave differently because .Items is now a collection and can be acted upon using standard array / collection methods (for...each etc) rather than needing special handling compared to an array or collection declared in code. These changes bring a consistency to the language / runtime that means you do not have to remeber each way of dealing with a similar but implemented differently object model / control every time a 3rd party or different intrinsic control is used.
Class names are now fully qualified with a namespace - this aides the logical groupiong of related classes and narrows down the IDE's intellisense when you know what area you are looking in. They also allow duplicate classnames to exist without causing ambiguous naming conventions. Imports are just a shorthand way of refering to these fully qualified class names, nothing more; as to how you know which ones to use and which references to make - experience with .Net and use of the help files would be a start. Unless you were born with intrinsic knowledge of VB6 how did you know to use MSCOMM for serial port access, InStr to find a string inside another etc? Whenver you learn a new programming language or upgrade to a newer version there will be a learning curve, and admittedly going to VB.Net from earlier versions can be a steep one but the benefits are there when the move is made.
Changes like requiring math functions to be preceded by the math class are a result of .Net being Object Orientated, functions are now grouped logically rather than just existing in one large global pool i.e. Math functions are Math., String functions are String., Enviroment details are Environment. etc.
In regards to the comm stuff (even though it wasn't your main focus) it took a quick search on google to find http://msdn.microsoft.com/msdnmag/issues/02/10/netserialcomm/