It doesn't exist a guide from HMC 4.5 to Exch 2010 SP2 cross-forest.
I suggest you use the following guide:
http://www.microsoft.com/en-us/download/details.aspx?id=28788
And use HMC 4.5 to Exch 2010 SP1 cross-forest guide as a secondary resource, you need to mix some steps.
Dario Woitasen | MCITP: Enterprise Messaging Administrator 2007/2010 | MCTS: Microsoft Lync Server 2010, Configuring