Here are some crucial points to keep in mind.
Features
Does the new software include all of the features that are commonly used in the old software?
Security
Is the software still in active development? If so, bugs can still be fixed.
Has the software been around for a fair amount of time? (A few years) If so, there is more chance that bugs have already been fixed.
Future
Is the software competitive with other software? If so, this implies the software is not dieing out.
Performance
Can the software provide fast page load with a huge amount of data, for different types of data? Such as accounts and posts.
Code Quality
Is the software using modern coding approaches, or messy outdated approaches? Tidy modern code is more secure and easier for both their developers and your developers to build features upon.
Customization
Does the software support customizing the design sufficiently? If so, this prevents rewriting their code.
Does the software support plugins? And is the plugin API good enough? If the plugin API covers a lot, there should be a wide range of plugins available.
Migration
It doesn't really matter if the software doesn't provide tools to migrate from old software to new software, because this can be done as a one off task manually by developers with several SQL statements.