Does changing the calendar from months to 4-4-5 weeks going to be okay? What are the issues with 4-4-5?

I don't think you'll run into any major problems with "core" Netsuite like posting entries, running reports, etc, though obviously year-over-year reporting will be less useful for the first year you're on 4-4-5. Caveat: I haven't dealt with 4-4-5 months for years, so I'm not sure if this still applies, but the things I remember having problems with (depending on which methods you use): 1. FAM 2. Amortization 3. Rev Rec a. Though on legacy RR b. Hopefully ARM fixed this 4. Allocations 5. 3rd party apps that might use posting periods Even if the issues around these might supposedly be fixed, I'd still test those modules thoroughly if you use any of them since they're kind of "add-ons."