I currently hold the Asterisk version 1.8 and wish to migrate to the platform AsteriskNOW. which would be the most viable way and step by step to follow the same
You are not comparing like with like. There are versions of AsteriskNOW that include Asterisk 1.8.
AsteriskNOW complete software to install on a bare machine and consists of: asterisk, the OS, dahdi, and an installer, together with an optional GUI front end for Asterisk.