This is the third of four questions posed to me by a journalist about open source and the public sector.
The economists have an answer for this. At any point in time should you evaluate the total life-time value of the various alternatives at hand and then chose the one that has the best value.
When making this calculation for a switch from proprietary to open source software, the switching costs have to be added to the cost of using an open source solution. It may well be the case that the open source solution, in itself, is much better than the proprietary solution, but with the switching costs added, becomes less desirable. Then, the rational choice is to stay with the proprietary solution.
This of course is maddening to open source enthusiasts, because a superior open source solution suddenly loses out to an inferior proprietary solution, because of the existing lock-in. However, this is simply economics.
Thus, the full answer is: It depends. In some cases, existing proprietary software should be replaced with open source software, and in other cases it should not.
It should be noted that switching costs are one-time costs, while cost savings through open source software are recurring. Thus, for a proprietary solution to keep winning over an open source solution, it must be significantly better and/or the switching costs quite high.