Th rule is (as I understand is you cannot go smaller than the base supplied with the model. But you can go larger - to the disadvantage of the player. You can quibble it's also a advantage... but it's marginal far outweighed by the disadvantage.
So increase by all means... just no shrinking. Also the metal terminators are still supplied n the small base. As for the oblits... it's the base supplied.
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