OK, so not strictly related to how to launch an intranet per se, but some of these recommendations will allow smooth transition and rapid uptake of the new intranet, which I think is tangentially relevant, yes?
from How do approaches for UX differ between intranets and normal websites?
there are a number of approaches or UX strategies you could employ, here's my thoughts from another question:
• Pay special attention to the intranet homepage; it is essential to attracting users and communicating information
• We need to gain a detailed understanding of internal workflows and processes so that we can presenting standard intranet information, including policies, forms, and procedures, much more-so than a standard website. Detailed user narratives will have to be build that define personas and test cases for use later in the project.
• There must be effective communication through the intranet: news, print publications, and email
• One needs to consider rules for writing news items for the intranet and what news items are likely to be most useful to staff
• The intranet will present information about the organization, individuals, and teams; allow user to find people
• The intranet will allow staff to Discover what people do and where they report; will be an effective employee directory
• They need to consider what to include in employee profiles, how to present team and project information
• According to the available research Nielsen Norman Group Report Series Intranet Usability Guidelines intranet search is a widely used and incredibly important feature
• The intranet should structure data by task rather than department
• The intranet must manage content and support users in multiple locations and using multiple languages.
• How will they manage the intranet? There must be content management roles, forms and processes which in turns means educating, managing, coordinating and training content contributors.
• Templates and standards will create a consistent experience
some of these things you will not have the resources for eg large scale, company-wide user research, and there is not much advice for how to implement or migrate staff to the new intranet, but these recommendations take little effort and can reap enormous rewards. Follow them and you will see that the new intranet will most likely be adopted by staff with minimal friction.