There is no simple small document that we can point you to for a migration. This will depend on how the data is structured in Heat and what you want to migrate over.
One thing you'll have to ask first is "is it even worth transferring/migrating data over?" Most of the times the answer there is No if you can have a legacy/read-only/reportering type of tool still running after adopting a new tool.
In general for such a project (cause don't underestimate it) you'll want to reach out to a solution partner who can advice you on how to analyse what you have and provide a solution/plan on how to move forward.
