Hello. I work for a company that has a custom application, developped on very old technology and they cannot maintain it or make it evolve anymore. So they want to have it re-developped  completely with newer technology and architecture. I have to write an RFP to find the provider who will custom develop it. However, the functional scope of this solution is huge and I do not know how to approach it.

Option 1: make very detailed requirements (including the many business rules) in the RFP so that the providers who respond to the RFP can estimate accurately the price for the development of the new solution. The problem is that it will take a very long time, and the very detailed business rules will probably change before we get to the developement phase in 2 years.

Option 2: make a list of higher level requirements for the RFP (and detailed requirements/specifications will come in the developement phase of the project so we are sure that business rules are up to date) but I am not sure the providers who respond to the RFP will be able to make accurate estimates of the price.

So today I am unsure how to proceed and I would love to have the feedback of other BAs on this topic.

Thank you very much