Well, a question I had was already asked and will probably be answered soon — the issue about what happens to the monthly fee of a Class 4 island that gets transferred — but I have another suggestion for you to think about:
Since the new Class 5 servers are supposed to be much faster and better overall — thus prompting LL to raise the monthly fee by 50% — would you consider to add 50% as many available prims to those servers as well? (effectively giving people the same amount of prims-per-dollar as elsewhere)
I know that the issue about more prims means more textures, and more textures is what crushes the clients and adds to lag. Still, this affects mostly the users with low-end computers (like myself :) ) which will experience lag basically anywhere — except, of course, on places where content creators take into account the average video card memory size and place a number of visible textures that will not overstep that boundary. The art of good content creation will always be better than increasing the speed of machines... so... the argument that a server that has more prims available is going to be dragged down to its knees if the prim limit is increased is only valid for those people that don't understand how SL works :)
By contrast, good content created on a Class 5 server will be able to serve lag-free 22500 prims as well as 15000 prims on a Class 4 server — assuming, indeed, that the Class 5 servers have 50% more performance than a Class 4 server, of course :)
Ian Linden's comment on this SL blog tends to indicate otherwise, ie. that the Class 5 servers would not have an increased prim limit, exactly because most people (> 99.999% of the resident population these days!) have no clue how to create proper content to deal with things client-side. However, one should also consider that we're talking about a price increase on private islands, not on the mainland. Owners of private islands should know what they're doing and the kind of experience they're offering to their visitors; if they can't manage to create quality content, they're the only ones to blame :) On the other hand, deploying Class 5 servers on the mainland is another story — since your content might be the best designed in the world, but all it takes is a few dozens of prims with different 1024x1024 textures on each side to lag that whole mainland sim (when your camera is pointing to that direction). Thus, the decision not to increase the prim allotments on the mainland would indeed make sense due to so easy potential abuse there.
But on private islands — there is really just one person to blame, and one person to complain to (by not going there!) if they don't know how to handle their extra prims :)
Please consider this option of giving private island owners on the Class 5 servers 50% more prims. It would make the transition to a much higher cost on the private islands more bearable. As a matter of fact, it would probably have lots of people rushing to buy the new ones to get those extra prims — as opposed to a slightly slower growth in the next few months until people realise that they don't have any other option than to accept the new fees if they want their private islands...
Unless, of course, this is part of the plan of having people give up their expansion on private islands and return to the anarchic mainland again :)