Hi
I would approach this way
1) Stand up the Lync infrastructure in the new forest.
2) Create "linked" accounts in the new forest.( users not residing in the target would authenticate via edge ). Later after the password cache entry is made end users wouldn't be prompted for the password ,since edge server allows communication
with federated companies.
3) Enable all the users in the new target forest (probably do it by batch to see the behaviour)
4) User DBIMPEXP to export their contacts, conf dir to the new forest. Use Importcsuserdata commandlets to import on lync 2013
5) During the final cutover date you can break the linked accounts (source account) by clearing "msRTCSIP-OriginatorSID" attribute and enabling the target account.
Very Imp Note :
This will be a disruptive migration and this needs to be planned properly in all phases and done over the weekend.
You cannot share the same address space between two different forests.