Your question has two aspects, so I will try to cover both.
Sitefinity v3 will be a major step forward and will not be backwards compatible with neither of the current versions. One difference is that it will be for .NET2 only. It will also offer membership services and masterpages - features which are .NET2 dependant and are not available in .NET1.x versions. Additionally, it will offer new layout rendering, i.e. it will not use tables any more, but rather DIVs and CSS. This is major change which we are not sure how to address yet.
Conversion from old .NET1.x to Sitefinity v3 projects
We are not sure that this would be possible but we will give it a try once v3 is out. Such converter will also depend on the number of Sitefinity customers who request to convert from .NET1 to .NET2. Nothing is certain at this time, so I cannot promise that we will provide such tool as this may not be achievable.
Conversion from old .NET2.x to Sitefinity v3 projects
We will try to provide a project converter from older .NET2 Sitefinity version to v3, but we expect to have troubles with the migration, so the tool may take some time to be developed. We will start working on the converter after the official release of v3.
I sincerely hope this information will help you with your purchasing decision. Do not hesitate contact me if you have other questions.