We’ve had some customers lately who thought they could just pick a data migration solution (typically Data import export framework, or DIEF), drop it in, and go.
While anyone who’s been in Dynamics AX 2012 for a while will tell you that DIEF is a HUGE step above anything that preceded it… the fact remains, it’s going to have to be something you plan. If it’s an afterthought, your project will certainly run late.
In fact, migration is so complicated that Microsoft created a matrix to help us decide which method to use – specifically because there is no one size fits all. http://technet.microsoft.com/en-us/library/aa548629.aspx
There is an old carpenter’s saying, “Measure twice, cut once” meaning planning before doing is the crux.
So migrate, like I vote – early and often!