One of the major factors which hinders the change management process is people's natural tendency for inertia. Just as in Newton's first law of motion, people are resistant to change in organisations because it can be uncomfortable. The notion of doing things this way, because 'this is the way we have always done them', can be particularly hard to overcome.  Furthermore, in cases where a company has seen declining fortunes, for a manager or executive to view themselves as a key part of the problem can be very humbling. This issue can be exacerbated in countries where "saving face" plays a large role in inter-personal relations.
Smaller requirements are easier to estimate. Shall statements, such as “the system shall convert feet to meters”, are an example of very small requirements. User stories are a little larger but still relatively easy to estimate. Use cases , a staple of the Rational Unified Process (RUP) and the Agile Unified Process (AUP) , can become too large to estimate effectively although you can reorganize them into smaller and more manageable artifacts if you're flexible. A good rule of thumb is that a requirement must be implementable within a single iteration. Scrum teams usually have month long iterations whereas XP teams often choose one or two weeks as an iteration length. Short iterations reduce the feedback cycle making it easier to stay on track. Successful teams will deploy a working copy of their system at the end of each iteration into a demo environment where their potential stakeholders have access to it. This provides another opportunity for feedback, often generating new or improved requirements, and shows stakeholders that the team is making progress and thus their money is being invested wisely.
Ironically up-front estimates are often motivated by the desire of organizations to reduce their financial risk on IT project yet in practice this decision not only has the opposite effect it also motivates you to increase your technical risk too. My experience is that when business people are educated on the impact of this strategy, and when they’re given other viable strategies for planning and governing IT projects, they soon choose to abandon up-front estimating. I highly suggest getting senior IT management and senior business management to sit down and talk with each other about this critical issue.