What you are describing is a common situation, and a cycle that repeats itself as businesses grow and change. As you point out, business needs evolve, sometimes quite drastically. So does technology. In a fast-paced, flexible environment like yours, it is not surprising that the right technology choice 5 years ago would now be outdated, complex, unstable and possibly, impossible (or very costly) to adapt.
Despite the high cost and risk associated with replacing core technology solutions, they do need to be updated periodically in order to maintain business relevance and competitiveness -- though arguably, as infrequently as possible. One issue is that technology initiatives are often poorly justified, with management setting artificial expectations for increased sales and job reductions. Poorly defined objectives and unreasonable expectations are some of the reasons why so many technology initiatives fail.
As you embark on this realignment exercise, the first and most critical piece of advice I would give you is to go into it with a very clear understanding of what makes you and your company different: your core capabilities, processes, know-how. Whatever it is that makes your business unique and gives it its competitive edge. It is tempting to select a replacement solution with the 80/20 rule in mind: "if the solution supports 80% of our needs, it will work". I would caution that most, if not every credible technology contender in your industry will in fact be able support 80% of your (and everyone else's) needs. Rather, the question is whether they can adequately support the core 20% that makes you different.
I would also make sure to select partners that fit your culture. If your employees are very autonomous and used to a lot of flexibility, do not implement a system designed around inflexible workflows, for example.
Without more understanding of your specific business and challenges, I can only provide input on generalities. I hope this helps.