A 3-option proposal helps a lot here. We do fixed-price upgrades, but we offer the customer 3 options to do it. The lowest option includes just the base upgrade, base report conversion, etc. It requires work during the day, and has a very short support period (like one day to confirm the basic functioning). No onsite (all remote). Specify support responses to be no shorter than 4 hours (you have other customers ...). It meets the definition of ""upgrade"" only barely.
In this case, next option would include a ""sandbox"" upgrade. Use a virtual image (yours or theirs) to install the upgraded system. Now you walk through with the customer to test all elements of the system. They sign off, so any further bugs are not on you. You then upgrade the production during working hours.
The third option includes the sandbox, the lets the customer choose to upgrade overnight or a weekend. Include all support for, say 2 or 4 weeks. Smother him in care. charge him for the security.
We price the 3rd option at ~3x-4x the base, with #2 in the middle, but closer to #3.
The beauty of this is that the customer gets to pick his poison, and you can, with clean and legal conscience charge him an arm and a leg for support if he chooses one and the gamble doesn't pay for him.
I think crm v7.2 is compatible with sage 100 2015. If so, do NOT upgrade them together. CRM is on its own server - treat it as a separate puzzle.