The OC should pay for a surveyor to work out what the new UIs should be and then, if you can get unanimous agreement, they will explain how you register this so you can redistribute levies on that basis. If you don't have unanimous agreement, you can apply to the CTTT for a judgement adjusting the levies accordingly.
Just be aware that there is a point at which the expense of all the surveys and legals etc outweighs the benefits of redistributing the UEs.
A more elegant solution, if the roof spave was common property, may be to lease it to the owners in way that doesn't interfere with their rights or their ability to sell but recognises the added value they have in their peroperty. That money could then go into the general fund and reduce your levies accordingly.