Migrating two seperate domains into one

I have one domain called solutions.local and another call intsol.interiorsolutions.net. I would like to have the solutions.local migrated to intsol.interiorsolutions.net with a single forest. This way I can properly implement DFS with R2.

What are my options without creating too much work for myself. The solutions.local is in Arizona (Remote office) and my intsol.interiorsolutions.net is (HQ) in SLC, UT. Servers are running Windows Standard 2003