Not that I know of.

 There are probably several areas that need to be changed:

1. 
   
2. Process definitions - If the user's name is explicitly used in any destination rules.
   
3. Process permissions - If the user's name is explicitly specified in any process permissions.
   
4. Workitems - You need to redirect all current work items in the old user's worklist to the new user's name.

1 & 3 would be non-issues if you are using Dynamic Destination Queues in the process destinations.  You can remove the old user name and add the new user name. The K2 server will take care of the allocation of the current workitems to the new user.

Thanks Johnny

I didn't write the processes (I wish I had), so I'll have to look through them all.

1. 
   
   I think there is at least one username explicitly in a destination rule, Dynamic Destination Queues haven't been used.
   
2. Process permissions are all group based, so that's okay. 
3. We are looking at quite a number of worklist items because a number of UserIds are being changed.

 I hoped that there would be an easier way to do it (updating the underlying SQL data perhaps) but I guess it will have to be mandrolic.