Deciding on the best approach to tackling legacy systems is one thing, but a major hurdle that needs to be overcome is building the business case for change. However logical it seems to a technical person, senior managers are rarely just persuaded by arguments that a new technology is better.
Other than a straight-forward decommission, all of the options for taming legacy systems will have a cost associated. Senior managers will want to know what they get back in return for investing the required cost. Many people try to justify the cost in terms of the efficiency savings that will be made. For example, replacing our legacy CRM system with CRM 2011 will save at least one hour per day for each sales team member.
Be wary of relying too much on this approach as it is hard to prove that efficiency savings really exist. Even if you can put the data together most senior managers know that efficiency savings don’t become reality. For example, you rarely reduce costs on the back of efficiency savings as something else comes along to spend the money on.
From my experience it is better to focus the business case on risk and reputation as key arguments for change. As any browse across the internet will show you there is plenty of high profile IT disasters that have ruined reputations. Whilst your legacy system might not be on this scale it is easy to lose the company money with unnoticed system problems.
So any business case that substantially reduces the risk of losses, either financial or reputational, will always be given serious consideration by senior managers.