The key factors are transparency and communication.
Some things you can do :
- organize workshops open to everybody to explain what it is all about and share the OKRs and nice stories of the roll out
- create a glassfrog account for "guests" to let them discover the governance, roles, politics... You can suggest them to follow any circle they want
- invite observers to meetings to discover the reality of the practice
- organize a survey in the Holacratic part of the organization (after a few months of Holacracy practice) and share widely the results and/or verbatim
Last thing coming to my mind : I think humility is key too. Don't try to convince, just show them the results !
Hope that helps