Do you prefer top down or bottom up change management?

it depentd on the work flow and objectives of the company..i prefer bottom up change

starts from the top, but it all depends on the objective though.

It depends on the objective of the change required, the nature of change and the level of significance to management. Normally, it should be top-down in my opinion. The management is in a better position to asess the objectives of change management, they are in a better position to communicate it downwards and have necessary influence to implenent the change. So I am in favor of top-down approach in most cases. But anyway it depends on the nature of change required.

A Top-Down approach is effective by itself mainly when dealing with critical situations, as it would be unlikely for the employees to be against the change at this point. Bottom-Down approach is more appropriate when innovations are needed, as this type of change does not normally have a time restriction. In my opinion, a successful change management is achieved by using a blend of the two approaches. Top-Down direction gives the business objective for performane improvements, while the Bottom-Down approach gets emplyees at all levels involved in achieving the objective. 

A bottom up change management is dependent on flexibility to accept the change.

A top down change management has an enforcement element attached to it by default. This enforcement is on the others who need to accept the change. Hence, even if there is a lack of flexibility within the hierarchy, it gets managed.