The better approach is to run things bottom up .. viz., document how you do business at the moment and then check that against the rules and recommended practices from authoritative sources. One then makes a much smaller change to the approach .. which is easier to implement.
I'm surpised to read that your company's SOP's havn't been updated for years but whatever the reason for that may be, refrain from completely re writing them as that would get you most certainly into big trouble with the supervising authority when you apply for approval.
Instead revise all relevant manuals thoroughly by today's standards and submit them to you CAA as amendments for approval otherwise you may risk suspension of your AOC pending approval of your new books !