It is possible to replace a non-tilt helm with a tilt helm.
It is most likely that such a procedure would only be simple for someone who was familiar with installation of steering systems and with the particular tilt helm that was chosen, had installed many of them, and had in that process learned exactly how to perform the installation in the most effective and efficient manner. That you inquire about the degree of difficulty suggest to me that you have not installed a tilt helm before, so, on that basis, I would assume that performing the procedure will not be simple for you to accomplish.
To establish the "type" tilt helm to be installed, you should refer to a vendor's or manufacturer's catalogue for details on tilt helms. SEASTAR SOLUTIONS is a common brand used. You can find more about SEASTAR SOLUTIONS tilt helms with this
link.
To become more familiar with conversion to a tilt helm, a good starting point will be to establish what type of steering system is presently installed on your boat and who made it. Then investigate with that manufacturer if a retro-fit to a tilt helm is available for the particular steering system presently installed on your boat. If a retro-fit is not possible, you may have to install a completely new helm and perhaps new steering system.
If the manufacturer of your non-Boston Whaler boat is still in business, contact the boat manufacturer to inquire about the details of the steering system that was used, and get their advice about the possibility of replacement of the existing steering system with a tilt helm.